This is everything you essentially need to know. In the last few years, the Firefox market share has fallen, a lot, most likely due to the rise of mobile and a failure of Mozilla to establish themselves on it.
The Firefox market share is such that it's practically unnecessary to test against it, especially if your users are using your web apps or sites on their phones. If you instead take that time and focus and create a more accessible app or site, it will be a more reasonable use of your time since more people have disabilities than Firefox has market share.
One billion people, or 15% of the world's population, experience some form of disability, and disability prevalence is higher for developing countries. One-fifth of the estimated global total, or between 110 million and 190 million people, experience significant disabilities.
Even for the most generous apps or sites, which only rely on desktop browsers for example and have users in regions such as Europe or North America, Firefox market share is lower than that. Of course, far from everyone with a disability has a disability that makes it difficult to consume web content but for most sites and apps today a majority of traffic comes from mobile users where Firefox market share is less than a percent which is lower than the percentage of people with sight issues.
Personally, I think the firing of Brendan Eich as the big boss was one of the biggest mistake Mozilla ever did and I think the current news about Mozilla proves me right. He had a salary of $800k as max and has done so much for the web as it is today. Suddenly when he was replaced, the salary more than tripled, isn't that strange? To request a salary more than three times as much as the previous guy should require the result being at least three times as good in my opinion. Mozilla has for the last couple of years shifted from being laser focused making the best, most awesome browser to promoting diversity over all else, purchasing pocket and pushing a lot of other things like Firefox Send. Things that have been good and bad, but most importantly not really about the browser or the web itself.
The funny thing is, that the urge to be profitable never seems to have encountered the most obvious realization, to profit from their most popular product, Firefox itself. They could simply charge for Firefox to unlock some pro features like ad blocking, Firefox Send (which was actually a great service) and other things that most normal users don't use anyway. Hence, they could keep their mission cake and eat it too.
I would gladly pay for Firefox if the money actually went there, but it seems that even donating to Mozilla is just sending your money towards some diversity cause or perhaps to Bakers own pocket.
To summarize my thoughts, it is clear that specifically supporting Firefox in its current state is not a goal for me anymore. I have lost faith in Mozilla after all these years. I will use the time to test for accessibility instead of being an active tester for Firefox. If you have the luxury to have enough time to always thoroughly test for both things, that's awesome but for me that is simply a position that I seldom find myself in. From now on, I will simply assume that stuff I make works in Firefox as I follow the web standards. Because honestly, if you have to choose it is likely better to test for accessibility than for Firefox.
For you people out there that values privacy but not political activism, I recommend you to try out the Brave web browser as an alternative to Firefox.