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If a headless Firefox screenshot has different pixel dimensions from the size you requested, check three things separately: Selenium’s browser-window size, Firefox’s headless virtual-display size, and whether you captured the current window or the full document. Set and log the Selenium size, inspect the saved PNG itself, and confirm the capture method matches the image you want. Those checks diagnose the mismatch without assuming that one historical report describes every Firefox setup.
Why the requested size and screenshot size can differ
“Resolution” can mean different things in a Selenium test. The size returned by driver.get_window_size() is the browser window’s reported width and height. Firefox’s headless virtual display has its own documented dimensions. The saved image has actual pixel dimensions, and its height also depends on whether you captured the current window or the whole document. These values are related, but checking one does not prove the others match.
A 2020 geckodriver issue report illustrates the distinction: its reporter requested and read back a 1024×768 window, but the PNG from save_screenshot() measured 1024×694. That report was for macOS 10.15.4, Firefox 78.0.2 (64-bit), geckodriver 0.26.0, Selenium 3.141.0, and Python 3.7. It is a specific historical reproduction—not evidence that current Firefox always loses 74 pixels or that the same cause applies to another environment. See the Mozilla geckodriver issue report for that context.
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Set and record Selenium’s window size
Set the intended size explicitly, then read it back after navigation or other setup that could affect the window. Selenium’s documented Python pattern is driver.set_window_size(width, height); driver.get_window_size() retrieves the current dimensions.
from selenium import webdriver
# Create the Firefox driver using your existing headless configuration.
driver = webdriver.Firefox()
try:
driver.set_window_size(1440, 900)
driver.get("https://example.com")
window_size = driver.get_window_size()
print("Selenium window size:", window_size)
driver.save_screenshot("current-window.png")
finally:
driver.quit()This example demonstrates the sizing and diagnostic calls, not a prescribed Firefox or geckodriver installation procedure. Keep the headless configuration used by your project and record its versions when investigating a discrepancy. If your installed Selenium binding or project uses different setup conventions, follow that binding’s API documentation; the issue described here concerns the dimensions, not a universal driver-construction recipe.
Check Firefox’s headless virtual display separately
When Firefox runs headlessly, determine whether your setup needs explicit virtual-display dimensions. Mozilla documents the environment variables MOZ_HEADLESS_WIDTH and MOZ_HEADLESS_HEIGHT for this purpose. Set them before starting Firefox, then create the driver and check its reported window size. Changing an environment variable after the browser process has started cannot configure that already-running process.
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import os
from selenium import webdriver
# Set before Firefox starts. Choose values for your test.
os.environ["MOZ_HEADLESS_WIDTH"] = "1440"
os.environ["MOZ_HEADLESS_HEIGHT"] = "900"
driver = webdriver.Firefox()
try:
driver.set_window_size(1440, 900)
driver.get("https://example.com")
print("Selenium window size:", driver.get_window_size())
driver.save_screenshot("current-window.png")
finally:
driver.quit()The environment variables and Selenium call address distinct documented sizing concepts. This example sets both deliberately so you can observe their behavior in your own supported configuration; it does not establish that both are required or that they override each other identically in every Firefox, geckodriver, and Selenium version combination.
Verify the PNG instead of inferring its size
A successful screenshot call only tells you that a file was written; it does not establish that its pixel dimensions equal the values Selenium reported. Inspect the PNG’s width and height directly. The following small Python function reads those dimensions from the PNG header using the standard library, without an image package:
import struct
def png_dimensions(path):
with open(path, "rb") as image:
header = image.read(24)
if len(header) < 24 or header[:8] != b"x89PNGrnx1an" or header[12:16] != b"IHDR":
raise ValueError(f"Not a valid PNG header: {path}")
return struct.unpack(">II", header[16:24])
print("PNG pixel dimensions:", png_dimensions("current-window.png"))Compare this output with the logged window size, keeping in mind that one describes Selenium’s reported window and the other describes the file. If your target is an exact viewport-sized image, also make sure the selected capture method is intended to capture the current window.
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Choose current-window or full-document capture
The ordinary WebDriver screenshot API captures the current window. A full-document screenshot has a different scope: it is intended to include the page beyond the visible viewport. Firefox’s Python API documents full-document screenshot methods, including get_full_page_screenshot_as_file. A taller full-page image is therefore not necessarily a window-sizing failure.
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from selenium import webdriver
driver = webdriver.Firefox()
try:
driver.set_window_size(1440, 900)
driver.get("https://example.com")
# Current-window capture:
driver.save_screenshot("viewport.png")
# Firefox full-document capture:
driver.get_full_page_screenshot_as_file("full-document.png")
finally:
driver.quit()Use the first method when the deliverable should correspond to the current browser window. Use the Firefox full-document method when the deliverable should cover the document beyond the visible area. Check the dimensions of each output separately; the two files are not expected to have the same height when the document extends below the viewport.
A practical diagnostic sequence
- Record the environment. Note the operating system, Firefox version, geckodriver version, Selenium version, and language binding. Historical reports are tied to specific environments, so these details help distinguish a reproducible configuration issue from an assumption based on an older example.
- Set and read back the window size. Call Selenium’s supported window-size method with the intended width and height, then log the result from
get_window_size(). - Check headless display dimensions. If the Firefox session is headless, review whether explicit
MOZ_HEADLESS_WIDTHandMOZ_HEADLESS_HEIGHTvalues are appropriate for your configuration. Set environment variables before launching Firefox. - Capture the intended scope. Decide whether you want the current window or the full document, and use the corresponding documented API.
- Measure the output file. Inspect the saved PNG’s pixel dimensions, rather than treating Selenium’s reported window size as proof of the image size.
- Reduce persistent problems to a reproduction. Keep the target page, sizing calls, screenshot method, environment details, and observed dimensions together. The historical issues do not establish one current fix that applies to all setups.
Common symptoms and what to check
| Symptom | What it may indicate | Next check |
|---|---|---|
get_window_size() reports the requested dimensions, but the PNG differs |
The reported window size and saved image dimensions are different measurements; the capture scope or headless display may also matter. | Measure the PNG, check headless display settings, and confirm the screenshot method. |
| The screenshot is taller than the visible area | You may have captured the full document rather than the current window. | Use the current-window capture when viewport/window output is the goal. |
The size remains unexpected after calling set_window_size |
The requested Selenium window size alone may not settle how a headless session is configured. | Read back the window size, inspect the PNG, and check the documented headless display variables. |
| A past workaround does not change a newer setup | The older issue may be version- or environment-specific; it is not a compatibility guarantee. | Record current versions and make a minimal reproduction before attributing the cause. |
Firefox command-line sizing is a different path
Mozilla’s Firefox command-line reference documents --window-size width[,height] for use with Firefox’s --screenshot command. Its scope is that command-line screenshot workflow. Do not assume that this option is a drop-in replacement for Selenium’s window API or the headless display environment variables; the documented controls apply to different interfaces.
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Or skip the browser setup
If your actual task is to obtain a page screenshot rather than test Firefox’s rendering behavior, ScreenshotNeo offers a one-request screenshot API. It accepts a URL and returns PNG, JPEG, WebP, or PDF. This is an alternative capture workflow, not a Selenium fix or a way to reproduce a Firefox-specific test.
For request parameters and response details, see the ScreenshotNeo documentation.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpScreenshotNeo removes cookie and consent banners, newsletter popups, and chat widgets before capture; each of those steps can be turned off. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and the response indicates the page verdict and billing status in headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for AI agents using Claude, Cursor, or another MCP client. The free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 screenshots.
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What the evidence does not establish
The 1024×768 versus 1024×694 discrepancy comes from one 2020 report; it does not establish how often the issue occurs, a universal 74-pixel reduction, or a single cause shared by current environments. A separate historical geckodriver full-screen report is likewise an example, not current compatibility guidance. Treat both as prompts to capture your own versions and dimensions, not as proof of a present-day Firefox defect.
Frequently Asked Questions
Does a 1024×768 window always produce a 1024×694 headless Firefox screenshot?
No. Those dimensions were reported in one 2020 setup; they are not a general rule for Firefox.
Can ScreenshotNeo reproduce a Selenium Firefox rendering test?
No. It provides a separate screenshot API and MCP workflow; it does not substitute for testing Firefox-specific behavior in Selenium.
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