It's easy to package a Firefox extension on Windows after you have all the files ready: just zip the files up with WinZip, and change the file extension to xpi. But on a Mac, you can't just change the file extension and expect the file type to change also. When I tried to drag a zipped file with the xpi file extension into Firefox, it asked me if I wanted to download the file, instead of trying to install the file. How am I supposed to package the extension, then? Do I have to switch to a Windows computer everytime I want to do so (Not that it happens often)? Is keeping the extension in the unpacked form in a special folder on my computer my only other option? I can't seem to find any references to this in the manuals for developers.

You could always set up a webserver on your mac and make sure it reports the mime type for .xpi properly.

I have a feeling your mac is being too smart for its own good and looking at the alternate streams on the file. Either that, or you just renamed the file without changing the extension; i.e., file.zip -> file.xpi.zip. Try doing the rename in the terminal as opposed to the finder. Maybe the Get Info command will let you change the file type...

I hope all of this helps

Packagin An Xpi on a Mac - How?
Packagin An Xpi On A Mac

You can go to Tools -> Add-ons and drag the .Xpi file. It will then ask you if you want to install that file.

I'm sure it works because I just did, though "tools" and "add-ons" might be with different names in your firefox, because mine is in Spanish, and that's the closest words I could think of.

-daniela