Is it disabling extensions as well as themes? I have not seen that. It does disable some of my themes that have not been updated for over a year or so. I have edited the install.rdf files to bump up the max version for the target application. When Firefox does that I simply reinstall those themes. I keep backup copies of all the installers for themes and extensions that I use.

Yes - it is the extensions - fireftp, firebug, zotero, and some others. They are automatically checked for compatibility, but I have to manually enable all of them.

This happens when the extension registry files are either removed or get corrupted and new files are created.

Delete the extensions.* files (e.g. extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar ("Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

If this hasn't helped then also delete the addons.sqlite file.

This can also be caused by problems with preferences.
Rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default value.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Thanks - this makes sense. I'm going to mark it as solved, although I guess I won't know until the next update.

Which security software (firewall, anti-virus) do you have?

Bitdefender

Thank you for taking the time to think about this.

I also have BitDefender Antiphishing Toolbar 2.0 installed as a Firefox extension, and I haven't been able to remove it. I get a message "BitDefender Antiphishing Toolbar 2.0 is incompatible with Firefox 22.0", but there is no remove button in the addons manager, and it doesn't appear in my profile extensions folder.

I believe it's an option on their install, so I could get rid of it by uninstalling BitDefender, and not checking that option on the reinstall.

Extensions that do not have a "Remove" button are installed by other software and are not under control of the Firefox extension manager.
Such globally installed extensions are usually found via a registry scan or are installed in a location that Firefox scans for installed extensions.

• https://developer.mozilla.org/en/Adding_Extensions_using_the_Windows_Registry
  

Extensions installed this way need to be removed via the settings (options/preferences) of the program that has added this extension or you need to uninstall this program via the Control Panel.
In Firefox you an only disable such an extension.

yes, I went down that road, followed the instructions here: https://support.mozilla.org/en-US/questions/759689

This wasn't successful. I believe I have to uninstall BitDefender and reinstall it, so I've just let it lie. Why did you ask about my firewall?

Some security software can protect files against changes and thus can prevent Firefox from finishing an update properly. So if you have problems after an update then these issues can possibly be caused by your security software.