This can happen if you try to install an extension or dictionary that only works properly when this extension is unpacked. Extensions aren't unpacked by default, but they should have an unpack flag in their install.rdf file in the XPI (ZIP) archive. * https://developer.mozilla.org/En/Updating_extensions_for_Firefox_4.0#XPI_unpacking ---- It is possible that there is a problem with the file(s) that store the extensions registry. 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 to check if there is a compatibility update available. See also: *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes