When you install an add-on in Firefox, you will download .xpi file. This XPI extension is default format for Firefox add-on. If you have many different computers, or you often work with different computer, you may need to install same add-ons for Firefox. It's better if you know where Firefox saves XPI files so you can backup them all
and install them on different computers. Firefox saves XPI files on
You'll find xxxx with some random alphanumeric characters. You may check it if you have installed some add-ons. Here, you'll find .xpi files have non-human readable names. So you'll need to open it on archive manager if you want to know what add-on behind the file name.
Note: to install an already downloaded XPI, just drag the file into opened Firefox window.