Should add the obvious ultimate way to prevent fingerprinting of this type is to just run Firefox in its own VM or container with a totally clean OS you otherwise don't touch. You could choose to share the Downloads folder between guest and host so you can still save files, but it then wouldn't be able to see what you do and don't have installed on your real host system.

Of course, if you do anything to allow hardware acceleration in your browser so you're not streaming media like it's 1999, it'll still be able to fingerprint you based on the hardware, but at least it won't see what applications you have.