It's about effective advocacy. Stallman can think or do whatever he wants, but he is not and has never been an effective advocate.
And from an advocacy position, ideological purity almost never works. An advocacy organisation like the FSF should be pragmatically "climbing the ladder" one step at a time, not insisting on an instantaneous revolution. You might as well advocate for the moon to be painted blue: it's just not going to happen, no matter how right you are.
You absolutely can achieve short-term wins without losing the long-term picture. The idea that any compromise somehow diminishes the longer term ideals is just wrong. Consider a law that states that purchasing software must come with the source code (under any proprietary license/NDA) as a matter of "right to repair". This would be a complete non-starter for ideological purists because it's not True Freedom, but clearly it would be a huge improvement over the current situation. This is how the ideological purity is impeding progress.
The reasons for Firefox's decline are unrelated to "compromised ideals" (which have IMO been hugely overstated in the first place). Quite frankly, no one cares about any of this outside of a relatively small group of people. If Mozilla had been more "ideologically pure" they never would have implemented DRM and would be considerably more irrelevant today. If you think "oh, this new browser has more Freedom (a concept you need to spend 10 minutes explaining in the first place) but you can't watch Netflix with it" is going to convince many people then you're sorely mistaken.
The fact is: Firefox started declining as Chrome was just a better browser. That's all there is to it. Firefox Quantum significantly improved things, but it seems it was "too little, too late". All the cases where Free Software is a resounding success is because it's the best possible software for the problem, and the cases where it's not is because it's not the best software for the problem.
Free Software will succeed if it's the best solution, so that's where the effort should go, not the (frequently minor or even petty) stuff Stallman keeps banging on about.
Edit: the fact that this got downvoted in less than a minute – hardly enough time to actually read the post – is also part of the problem. I don't really care about downvotes, but it's indicative of the kind of hostility that makes these kind of internal community discussions hard. My article that I previously mentioned got posted to HN, where it was promptly flagged[1], seemingly because some pro-Stallman people were upset by it. Fine, whatever, I didn't even care enough to email dang about it, but it does make these discussions a lot less constructive than they can be. I received more emails and private communication about this article than everything else on my website combined, almost all positive by people who appreciated a more nuanced take than "Stallman == atrocious" or "Stallman == secondComing(Jesus)", so clearly it wasn't all that bad. But on HN it was "censored" – for lack of a better word. I'm not directing this at you – your comment was constructive and fine – but in general it can be hard to have these discussions.
[1]: https://news.ycombinator.com/item?id=26579225