[nirvdrum]

Your assumption of what Fitts's Law is isn't quite correct. Wikipedia has a description of Fitts's Law [1], some of the follow-on work in that space, and its application to HCI. There's a multitude of books, papers, and blog posts that deal with the application of Fitts's law to UI design and usability, so I'll let you pick your favorite source and go with that.

In broad strokes, the time to accurately complete a task is proportional to the distance to an item and inversely proportional to the size of the item. So, having to scroll tabs increases the distance -- potentially a lot. But, constantly shrinking tabs reduces the target size. I think you'd have to do an amortized analysis on this one because upon a the creation of a new tab the Firefox model only makes that new tab harder to get to, while the Chrome model shrinks all tabs, making each harder to click.

But, I could buy that there's some threshold here where Chrome maintains a tab size that still doesn't appreciably increase error rate while improving distance by eliminating the tab scroll. I'd expect that minimal tab width to be much larger than Chrome's true minimum tab width. But, I'd be shocked if Google didn't run usability studies on this. I just haven't come across anything on my own.

[1] -- https://en.wikipedia.org/wiki/Fitts%27s_law

[olah_1]

Based on my own experience, I think Chrome hits the nail on the head because usually the only helpful thing in terms of finding the right tab is (1) the favicon and (2) the general position of the tab among the other tabs.

General position is particularly helpful because, from left to right, it roughly represents "time opened", earliest to latest.

Firefox doesn't help me find the right tab just by including an 8 character width along with the favicon. Might as well just get rid of those 8 characters.