How do I enable software installation in Firefox?

I’m trying to add an extension, but I get the yellow bar at the top that says “Software installation is currently disabled. Click Edit Options… to enable it and try again.”. What the hell am I supposed to do in Options to enable it?

Save the file to your desktop then run it from there. Firefox disallows directly installing .xpi extensions from web pages for security reasons. You can whitelist Firefox extension sites and it will install those automatically if you want.

Under the Options->Content tab, you’ll find a section that says “Warn me when websites try to install extensions or themes”. If you go into Exceptions, you’ll see a list of sites you’ve allowed or blocked. I believe that’s what you might have to change (I have a short list of four ‘allowed’ sites).