This is often caused by a corrupt file, to fix it you need to delete 3 files. In the Help menu select "Troubleshooting Information", this will open up a new tab. Near the top of the troubleshooting information tab, under "Application Basics" click on the "Open Containing Folder" button, this will open the profile folder in Windows Explorer (or your default file manager). Now close Firefox, then delete the following 3 files in the profile folder, extensions.ini, extensions.cache and extensions.rdf. When you re-start Firefox those 3 files will be recreated. You should then be able to install the extension.