I have been trying to install add-ons such as Firebug, Enhancer for YouTube, and TinEye Reverse Image Search from https://addons.mozilla.org/en-US/firefox/extensions/ as well as from the Add-ons Manager. However, in the case of the Add-ons Manager, I get the message "an unexpected error occured during installation". When I try installing an add-on from the url mentioned above, I get the message "[add-on] could not be installed because Firefox cannot modify the needed file". Unfortunately, I don't think this error message is included in the "Unable to install add-ons (extensions or themes)" page of the website. I took a look at the Profile folder under Application Basics in Troubleshooting Information and searched for files with the name "extensions". I followed the instructions for corrupt extension files and was only able to find and delete a file named "extensions.json". Upon restarting Firefox, I was still unable to install any add-ons and a new "extensions.json" was created. Also, while on the add-on page (url mentioned above), I checked the Permission section of the Page Info and the radio button "Allow" for "Install Add-ons" was checked.