Popular browsers support tabs. When you have many tabs open, it's hard to
show a meaningful title for each one. An icon takes up less place and is
easier to scan for visually.
Mozilla Firefox doesn't shrink tabs any further, but instead lets the tab list go off screen and you can scroll. I think that is a Google Chrome specific thing.
I've just tried and when I open a bunch of new tabs, Firefox truncates the
"new tab" text to "new" and a Firefox logo. Same thing happens with other
titles.
(Then at some point it stops truncating and scrolls off the screen.)
That's true, it's more without the favicon. It is configurable with browser.tabs.tabMinWidth. Not sure if it is configurable elsewhere in the UI, I normally don't bother with that.