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.