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It's similarly highlighted in both FireFox and WaterFox -
I've never had the problem so I never noticed (highlight is more extreme as a link, visually than the lack of a border).
On the other hand, I understand the illogic you bring up. For a custom tabbing page at work, my users see highlighted connected tabs. Again, I leave as little to chance as possible when "users" are involved.
I went with pale moon which fixed a lot of things mozilla broke in ff (fixed or didn't do)
... mostly the < save / quit / cancel > when you close the thing with lotsa tabs open.
I'll admit though my bank doesn't like pale moon, so I installed chrome just for that.
(dislike chrome, not just tabs on top of the address bar but no menu bar because that's "modern".)
after many otherwise intelligent sounding suggestions that achieved nothing the nice folks at Technet said the only solution was to low level format my hard disk then reinstall my signature. Sadly, this still didn't fix the issue!
It's displayed vaguely lighter, but when you have lots of tabs open (15+ is normal, for me), it's barely noticeable.
The "dumb Win10 default settings" have never been a problem for me (although I think "dumb" is a far too forgiving word, so you're probably a much nicer person than I am). I hack the cr@p out of the registry, to get rid of them.
But it's so bloody illogical, is what gets me. The tab is part of the page -- in the real world, it is physically part of the page -- so attaching it to an address bar makes absolutely no sense whatsoever, and is completely unintuitive.
It makes it into just a horizontal row of buttons that switch between pages that are somewhere else in the window, which is most certainly not good UX -- if you insist on having such buttons, at least align them vertically.
I wanna be a eunuchs developer! Pass me a bread knife!