I also received a message at first that the extension was "incompatible with Firefox" but then something made it work eventually. The extension is disabled each time I boot, and sends a verification code to my computer to enable. This experience is a bit strange given I'm already authenticated. I also had to enable a 4-digit pin to my computer to use iCloud Passwords, which seems less secure than a password and probably a dealbreaker for most. Aside from the setup and quirks, it otherwise works well and syncs immediately with my other Apple devices. I imagine the venn diagram of Firefox PC users and iPhone users overlapping is really small, so I appreciate that this even exists.