Delete the files extensions.* (extensions.sqlite, extensions.rdf, extensions.cache, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry. New files will be created when required.

See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar to do a compatibility check.

You can also try to reset the preferences:

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

After not achieving success in the "Run" section, I tried starting FF with add-ons disabled and that solved my problem! While in that mode I removed many add-ons that were not compatible with FF 4 - then restarted FF - all is now working properly as far as I can see. Will come back if I find anything else.

Aaaaargh - now the icons keep shifting around and worse, disappearing from everywhere including the "customize" option. What NOW????

See:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Create a new profile as a test to check if your current profile is causing the problems.

See:

• Basic Troubleshooting: Make a new profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files) See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox