There's a theme that I love (https://addons.mozilla.org/en-US/firefox/addon/ft-deepdark ), but it's a pain to update because it's not officially supported on OS X. In the developer's FAQ, they state: "It is not officially available for MAC, but for those who would like to try it anyway on that platform, you can force download (regardless if it is the Windows or the Linux file) and once the package downloaded on your computer, you can install it manually by clicking the 'tools' button in the add-ons manager and then selecting 'install an add-on from file...'." However, this options seems to have been removed in newer versions of Firefox. So basically, every time there's a major version release, I have to download the theme on a Windows machine and transfer the xpi file to my Mac.