It is possible that there is a problem with the file(s) that store the extensions registry. Type '''about:support''' in the address bar and press enter. Under the page logo on the left side you will see '''Application Basics.''' Under this find '''Profile Folder.''' To it’s right press the button '''Show Folder.''' This will open your file browser to the current Firefox profile. Now '''Close Firefox.''' Delete the extensions.* files (e.g. extensions.json, 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. After, restart Firefox. See "Corrupt extension files": *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.org/kb/Unable+to+install+add-ons