:-O -without being a developer

Hi there, I've spent 2,5 hrs trying to add the google analytics opt-out extension manually to no avail, i.e. doing so by downloading the .xpi file from google and then by trying to upload it as an extension added from a local file. It had already taken me over an hour to figure out how to even add an extension from a local file since that option isn't available in my latest firefox version by-default (I read that people need to downgrade the firefox version to be able to get to these extension settings!?). All in all, I finally managed to figure out I had to 'debug' the settings by clickling 5 times on the firefox icon (?!?!). When, at this stage, I managed to see the 'add extension from a local file' option on my "extensions' settings" menu, and try to upload the .xpi file, firefox would close and send me a warning message about it. i.e. the .xpi file wasn't uploaded, the firefox app was closed, and hence the extension wasn't set. I have managed to find the extension on mozilla add-ons website but, I'm still stuck with mozilla and .xpi files. How can I then upload an .xpi file to firefox? :-O -without being a developer Or add any extension from a local file to firefox (usually a .xpi file)? Thanks.