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
~/.mozilla/firefox/xxxx.default/extensions
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.