Joplin webclipper does not work if resistFingerprinting is set to true. Privacy is one reason to use Joplin over something like Evernote. Privacy is also one reason to use Firefox over something like Chrome. Fingerprinting is a particularly insidious form of tracking and the ability to limit it should be supported.
resistFingerprinting is not meant to prevent fingerprinting. As the name implies, it's for resisting fingerprinting, by obfuscating info or reducing the number factors sites can use for fingerprinting, but you probably know that. The fact that it breaks websites is my issue to deal with and it's an issue that I can live with. Yes, it's still a thing, and I suspect it will be so long as fingerprinting is used as a means of tracking.
And it also appears to break the Joplin Web Clipper.
I'm certainly in favour of such technologies in general but it seems Firefox is doing this on their own here with no support from other browser vendors or websites in general, so I expect they'll drop it or stop maintain it at some point. If it turns out it becomes an important option though we'll reconsider trying to make it work with the clipper.
In the meantime I feel there are already many of ways to reduce fingerprinting without breaking websites.
I might be misinformed, but I believe ResistFingerprinting is mostly for the benefit of the Tor Browser Bundle; which also has the policy of not suggesting adding non-default extensions anyway because each extra one becomes one additional fingerprinting factor.
If Mozilla's view is that the settings primary purpose is to help Tor, and Tor views extensions as incompatible with its goals, I'd have to assume Mozilla are unlikely to make changes to it, unless they benefit Tor directly.
I might be misinformed, but there's a reason it's in the about:config menu and not the user visible settings page; anything that's not directly exposed is considered unsupported generally.