Do you have any idea why all of a sudden my Firefox window becomes smaller after it first starts up at maximum window size? I have made no changes to the settings of Firefox?
This issue could be caused by a few different factors. First, try checking if any extensions are interfering with Firefox's normal behavior—start Firefox in Safe Mode to see if the problem persists. If it doesn't, an extension may be the culprit.
Another possibility is a problem with your user profile. You can create a new profile to see if that resolves the issue. Lastly, consider updating or reinstalling Firefox if the problem continues, as it could be a glitch in the current installation.