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.

Do tabs in the popular graphical browsers display a number on each tab by default

This might be useful when switching from, e.g., tab#1 to tab#7, using keyboard shortcut Ctrl-7