Constantly dinking with the UI and constantly making broadly unpopular, if not harmful (re. cliqz and hiding it) changes against any and all criticism. Mozilla at some point decided they know better than their users what their users want.
I'm not sure how you can say if they're broadly unpopular - certainly you mostly hear complaints but that could be because the only people bothered enough to write about it are the ones who don't like it.
I'm personally a happy user and like almost all the changes they've made over the last few years. I like the new UI. I haven't felt the need to post anywhere about being roughly content with it.
The market share stats looked bad long before they started messing with the UI, no?
I also was a happy user for like 15 years, I used to actively recommend it to everyone, but they keep removing the features I like. They broke extensions, removed bookmark descriptions, RSS, a "smart" omnibox that made it harder to find what I use the most, the recent changes to tabs. I still keep it as default because I'm used to the dev tools, but they just keep making it worse, I'm not surprised they're still losing market share.
Google, Microsoft, Apple, Samsung, etc pushing their browsers through their platforms would be the main reason. Most people don't care about the UI changes.
Yes it was a little jarring at first. I've been using the new ui for a while now and likely wouldn't want to go back (I tried the ui hot fix for a bit, but ended up removing it and going with the default).
Most people don't care about the specific UI changes. But they feel frustrated and helpless when they have to relearn what they knew suddenly with no benefit to them.
Things like lower contrast aren't just about taste or getting used to it.
...their market share stats show they were wrong.