I have 2 versions of FF, latest 14.x and also 4.0.1 which I want to keep. I start both with different profiles using
path_to_firefox.exe -P profile_name -no-remote
With 14.x I have no problems, but can't install any add-on in 4.0.1 - I tried installing from web or from a .xpi file - in both cases the download progress bar fills to 100% and then stays there forever, without proceeding further.
Is there a way to manually install an add-on? By this I don't mean from .xpi file, but really manually - if I extract .xpi contents, where should I place the files, and which other files I have to modify to have an add-on working?