Putting scrolling in a tab bar doesn't allow the user to take full advantage of spatial and muscle memories.

And when those memories fail, because scrolling tabs literally removes tabs from view, the user can't even revert to a visual search to find them! Instead one has to slowly scroll through a list of identically sized tabs, hoping that window #5 is actually the one containing the desired tab in the first place.

Oh dang, it was actually window #4.

Compare and contrast this to non-scrolling tabs, where tab size continually shrinks as tabs grow in number. #1 - you can identify which window is relevant by the tab sizes alone (so even if you move your windows around you're not lost). #2 - you can identify tabs by spatial memory, because you know approximately where, horizontally within the window, your tab exists, cutting your search space down to a few tabs only. #3 - you have visual cues as to when you might need to split out into a separate window for ease of tab management.

Of course, 1px is hyperbole, but 15px-30px is most certainly not, and on a modern display you can fit very many 30px tabs in a row without scrolling.

That's not to say that there aren't limitations to shrink-to-1px tabs, of course there are. But in all circumstances as a UI tool they are objectively better than scrolling tabs, until they shrink to an unusable size. At which point, scrolling tabs still isn't a good solution to the problem (grids, multiple rows, tab lists, tab groups are all vastly better options here).

Of course, if you don't have scrolling tabs then the tabs get so small that it's basically impossible to find what you want.

I solved this dilemma by hiding my FireFox tabs and using Tree Style Tabs. I find it really helps with keeping tabs organized and I can keep a lot open without worrying. I removed the scrollbar and close buttons, and made the tabs smaller, to further save on screen real estate.