OK, this morning when I opened FF, it started up with maximized window. Now when I open FF, it shows the maximized window for about a second or so, and then automatically becomes smaller? I have not made any changes to the settings, so I have no idea why it happens. It is a small issue, but it is annoying.
Window size of FF after a second.
It’s usually one of two things: Firefox restoring a saved window size from its profile, or a privacy setting (fingerprinting protection) that forces the window to a specific size.
You can try the following in order:
1. Quick test in Troubleshoot Mode
This checks if an add-on or customization is doing it.
- Open Firefox.
- Click ≡ Menu > Help > Troubleshoot Mode….
- Click Restart and then Open when asked.
Now close Firefox and open it again a couple of times in Troubleshoot Mode.
If the problem
doesn’t happen in Troubleshoot Mode, then most likely:
- An extension
- A theme
- Or a userChrome/user.js tweak
is changing the window size. In that case, try disabling extensions one by one, or undoing any hardening/user.js changes you’ve recently applied.
2. Check fingerprinting protection (if you use hardening/user.js)
Firefox’s
privacy.resistFingerprinting option can resize the window shortly after startup, so it looks very similar to what you describe.
- Type about:config in the address bar and press Enter.
- Click Accept the Risk and Continue.
- Search for: privacy.resistFingerprinting
- If it’s true, double‑click it to set it to false.
- Restart Firefox and test again.
If you see grey “letterbox” borders around the content, also check:
- privacy.resistFingerprinting.letterboxing – set to false to test.
3. Reset Firefox’s stored window size (xulstore.json)
Firefox keeps window size/position in a profile file. If that gets corrupted, you can see weird resizing like this.
- Close all Firefox windows.
- Press Win + R, type:
%APPDATA%\Mozilla\Firefox\Profiles
and press Enter.
- Open the folder that ends with .default or .default-release (or whatever you normally use).
- Find and delete the file:
xulstore.json
- Start Firefox again, set it to maximized, then close and reopen to see if it now stays maximized.
4. Double‑check the shortcut you use to open Firefox
If you always launch from a shortcut:
- Right‑click the Firefox shortcut you use > Properties.
- Under Run, choose Maximized, click OK.
- Open Firefox a couple of times and see if it still shrinks after that first second.
If it’s still happening
Post back with:
- Your OS version (e.g. Windows 10/11).
- Your Firefox version.
- Whether you’ve used any hardening/user.js (e.g. arkenfox).
- Whether Troubleshoot Mode (step 1) has the same behaviour.
With that info, people here can suggest more targeted steps.
