To add on to the other replies, you *can* load unsigned extensions with desktop Firefox if the build you're using disabled the signing requirement at build time. A bunch of distros' FF packages do that, for example, and is why I use a bunch of extensions I wrote myself (and thus trust) for myself without having to deal with Mozilla. (Zip up the files, change the file extension to `.xpi`, drop it in `$libdir/firefox/browser/extensions/`)
