Future? As a webdev I don't remember having to check if something works on Firefox since probably 7-8 years at least. Userbase is too small to justify allocating resources.
Too small? According to this site [1], market share is about equal to Safari and Edge+IE. If you are supporting Safari, Edge/IE there is no justifiable reason not to support Firefox.
I never did when I was maintaining an embedded web app. I checked in Chrome and Firefox. I would go in and figure it out if someone reported a bug in Safari but mostly no one at the company used Safari so it was really not tested for and the app just was never meant to run on mobile at all so safari wasn't much of concern.
[1] https://kinsta.com/browser-market-share/