[sirn]

I've been wondering about this for a while: what's with the large back button? Back when it was introduced, it was cool because back button was the most often-use one of all nav buttons (next, reload, stop, home) but now they're reduced to one, large back buttons just doesn't seems to make any sense anymore.

Are they still doing it as a part of Firefox UI identity or is there any (recent) usability research on this?

[Zirro]

The forward-button still appears when there actually is a link to go forward to (instead of having it greyed-out a lot of the time). My personal opinion is that it looks good, and that the round back-button has become a way of recognizing Firefox. The option to shrink it isn't that hard to find though, you just choose "Small icons" in the UI-settings.

[sho_hn]

You kind of allude to this with the identity bit, but to spell it out I remember reading in their docs that part of the original motivation for the large button was the idea to establish a cross-platform "keyhole shape" element (formed together with the forward button) there to identify Firefox by.

[Avshalom]

Branding aside it's usual a good idea to make the most used elements of the UI big, it makes them easy to click.