On desktop, various Firefox updates would change my settings subtly and nag me about features I had no interest in. On top of that, it was simply a slower browser, both with DOM manipulation and Javascript.
For example, when I made my latest research project (https://pl.aiwright.dev) Firefox would routinely struggle with the large complex graphs (over 100k nodes, though only a few hundred nodes attached to the DOM at any given time). Safari handles it with no issues whatsoever. I'm sure there are likely workarounds to speed things up on Firefox, but I don't have the time or energy for something like that since it's a research project, not an end-user facing product.
Maybe you can make the argument that switching to Firefox will incentivize improvements, but so far that hasn't been my experience. I've now moved away from Firefox to things like Orion (which is still quite buggy, but useful enough). Safari seems to be getting better over time, with things like containers now and extensions like Userscripts [2].