Tab close buttons are on the right (windows style) instead of on the left (macOS style), for one.

I mean, it’s the same on every browser except Safari

Just because everybody does it wrong doesn’t make it right.

Orion also does it right.

Every other browser aren’t using macOS native UI therefore are wrong in this environment. But right on their own in being the same everywhere.

Unfortunately “being the same everywhere” only works if your application is distinctive enough in functionality that it can’t be anything else but itself. Firefox is just a browser. Safari is also a browser. Chrome is also a browser. And none of these apps are more important or relevant than the operating system they are working under.

If my OS says windows and tabs close on the left, then I expect every single app to be a good platform citizen and not make me relearn default controls just to operate simple functions. Windows and tabs close on the left. End of story.

Not only that, but general colours are system-like but offputtingly different (dark theme is different from Mac dark theme just enough to be unsettling). Spacing (padding, margins, etc) is uneven on a lot of options, toggles and fields. Swipe back/forth being a simple arrow on the corner of the screen instead of full page swipe (like Safari) in the year of our lord 2026 is a complete disgrace (yes, chrome does this too, yes chrome also sucks). Press CMD+F and just watch as a search bar that hasn’t been updated to modern visuals in about 20 years pops up at the bottom. Pretty much everything around Firefox feels like a random collection of Windows and Linux behavioural patterns mashed together like a digital Frankenstein monster.

Firefox is not a bad browser by any means, but it feels like Firefox is the browser equivalent of the “how do you do fellow kids” meme. An out-of-time experience pretending to blend in under the guise of “AI”.

Most of those are subjective details and vary from platform to platform. Fine adjustments for each platform isn't guaranteed to attract many people to Firefox either. After all, the UI/UX of Firefox is good even if it isn't excellent. Factors such as convenience (e.g. familiarity or being pre-installed) are likely bigger factors. That said, I will grant you that things like the visibility of the find bar is poor.

It is also worth noting that the macOS UI/UX isn't anywhere near perfect. It is a hodgepodge of design decisions that go back to the origin of OS X or, in a few cases, the origin of the Macintosh iitself. Padding and margins may help achieve visual balance, but it's Windows management paradigm is an absolute functional mess.

Your argument boils down to “but Mac’s HIG isn’t perfect and UX/UI criticism is nitpicking”. OP is right though, UI/UX matters and despite Mac’s HIG being a mess, it is the Mac’s mess and users expect a Mac-like mess, Firefox should not be taking things into its own hands either way.

Maybe I shouldn't have included that second paragraph, since that's not my point. The second paragraph was more of an expression of frustration with how elements of the Apple user base regard macOS as the pinnacle of user interfaces and how everyone places an extremely high importance upon even minor elements.

Then again: while I believe bad UI/UX is a thing, I don't think there is an ideal. Different people think in different terms, or have different preferences. Some forms of interaction are better in one domain than another. There are undoubtedly many other reasons.

As for Firefox taking things into their own hands, that's the developer's decision. I have already given two reasons why that may be the case (users may expect consistency across platforms, and it may require too much effort for too little return).