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

Some security software does have virtualization or sandbox features that may cause problems by protecting and restoring files in the Firefox profile folder.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) 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, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

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

i deleted all that you have said to delete but it has not solved anything and yes i purchased an upgraded version of avast around the time i updated to FF13. ay other suggestions?

Try: Avast -> Additional Protection -> Browser Protection -> under Mozilla Firefox, uncheck the "Run this browser sandboxed" checkbox

thank u sooo much. your suggestion has helped and everything is back to normal

You're welcome