1. The fact that they look like buttons now, not tabs.
2. The active and inactive tab colors are so close that it's difficult to tell at a glance which is which. Especially in low-contrast situations (e.g. outside) or for those with vision disabilities.
3. Massive amounts of pointless padding. Which means you can't fit as many tabs in the window, and makes the browser a challenge to use for those with small screens. It also wastes horizontal space which is precious now that all monitors are widescreen.
4. The most important reason: they tried to fix something that wasn't broken and made it worse instead. There was nothing wrong with the Firefox UI but plenty of things they could improve where it comes to performance, extensions, HTML/CSS/Javascript behavior, and the large stack of open bugs in Bugzilla that have gone years without any attention.
Instead, they paid someone (or possibly a team) hundreds of hours to fluff up the UI.
Yeah, that's a bit annoying, but it goes away if you use vertical tabs through an extension like Sidebery. Tbh I stopped noticing it after a couple of months.
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.
