This page has links to user guides. Maybe one of those will help. https://agilebits.com/support

This will happen if you install an Add-on that needs to be unpacked in order to function properly. Normally Firefox installs the XPI file and modifies its name, but if an extension needs to modify a file in its file structure or otherwise only works when unpacked (e.g. a dictionary) then you can get this error if the add-on doesn't get unpacked. Extensions that need to be unpacked should have an unpack instruction in its install.rdf file: <em:unpack>true</em:unpack>

You can temporarily toggle the extensions.alwaysUnpack pref to true on the about:config page to see if that helps to install the extension.