Firefox won't install any addons/themes, existing ones don't show up on addon list.
Okay so when I go to tools>add-ons the list is empty, it shows no extensions, themes, languages, plugins even though there are ones installed and in use. Also when I try to download add-ons they don't install and don't show up on the list either.
Every time Firefox opened
Delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry. See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
If you see disabled extensions that are not compatible on the next start in "Tools > Add-ons > Extensions" then click the "Find Updates" button to do a compatibility check.