Hi macmozz, You can disable fullscreen from the about:config page, but to change it back you will have to follow the same instructions below:

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.
3. Please search for "fullscreen"
4. Setting autohide to false

If firefox is not behaving it may be a corrupt localstore file. More information on this can be read on herehttp://kb.mozillazine.org/Corrupt_localstore.rdf

The browser.fullscreen.autohide pref merely control whether the toolbar are hidden (i.e. the check box in the right-click context menu: Hide toolbars) and has nothing to do with preventing full screen mode.

On Mac you toggle full screen mode with Command+Shift+F.

• https://support.mozilla.org/kb/how-to-use-full-screen

I'm don't know a reason why Firefox would switch to font size randomly or unintentionally either then via the hotkey or the View menu.

This can be caused by a problem with localstore.rdf as that file stores the full screen setting.

You can also check for issues with the Firefox plist file and remove this plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

^thank you cor-el I learned something.

Thanks for the answers. They made me realize I used inaccurate terminology.

It's not fullscreen mode that's the problem. It's that Firefox sometimes opens zoomed to its maximum size. Like using the "Zoom" command in the Window menu (on a Mac), except I hadn't used that command, and it wasn't previously zoomed to max.

Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl+0 (zero); Command+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

Window zoom, not content zoom (i.e., on Window menu, not on View menu) Thanks

And fixing it when it happens is not the problem. The goal is to prevent it from happening unexpectedly and unpredictably.

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)