https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt xulstore.json file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/Profiles
• Linux: *~/.mozilla/firefox/Profiles
• Windows: %AppData%\Mozilla\Firefox\Profiles
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles
• Windows XP: C:\Documents and Settings\<user>\Application Data \Mozilla\Firefox\Profiles\

Search for the above file. Then rename or delete it. Restart Firefox.

Thanks for replying but there is not a file with that name residing om my computer.

Cheers!

I fixed it myself by deleting Firefox completely, reinstalling it, and then refusing the automagic update that started the second I ran the new Firefox.

All's well!

Sigh, and then right after I rebooted, it went right back to doing the same stupid thing it was doing before. And then, while I'm writing THIS RESPONSE I get pop up after popup from Mozilla wanting to make sure this was my default browser and others that ask to make my browsing experience EVEN BETTER. Hell, it wouldn't even let me try to install Firefox 88. (I'll try anything at this point) It's like Mozilla got secretly bought out by Micro$oft. At the very least I think Mozilla has forgotten who owns the laptop I'm currently using.

and then refusing the automagic update that started the second I ran the new Firefox.

Sigh, and then right after I rebooted, it went right back to doing the same stupid thing it was doing before.

Unfortunately, while Legacy Firefox settings allowed you to not update the browser, this was removed from Quantum Firefox. Why? We don't know.

Step one: How do you completely turn off update checks in Firefox v64? You can use a policies.json file to disable updates for all users. https://github.com/mozilla/policy-templates/blob/master/README.md or https://winaero.com/blog/disable-updates-firefox-63-above/

You can use a policies.json file in the distribution folder in the Firefox installation folder instead as also listed in the article, so no need to use GPO keys in the Windows Registry. • https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

Step two: Note: Some versions of Quantum change how the data files are set up, so they may not be backward compatible.

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements