Firefox 4 is out. There are plenty of comprehensive reviews out there, so I'm just going to look at a couple of features that caught my eye. These are features Opera has had for a while, so I'm going to compare Firefox 4 to Opera 11 (on Debian Squeeze)- it's FOSS Vs. Proprietary.
Firefox 4 has removed the menu bar and replaced it with a drop down menu button in the tab bar- this is certainly an idea I like as it gets rid of some wasted screen real estate. This is how the Firefox 4 and Opera 11 implementations compare.
I think Opera has the edge here- its menu button is neater, and the tabs more readable on a dark theme. (The Firefox menu button doesn't seem to give access to all the options present in the menu bar, for example View>Sidebar, so it looks like it's functionally less polished as well.)
Firefox 4 also introduces tab management, called Panorama- a feature Opera 11 already has in tab stacking. Here's how they compare:
For me, the Opera method is better. Firefox's Panorma requires you to fiddle around with the size of tab group windows, whereas in Opera's tab stacking, thumbnails are automatically grouped and displayed when hovering over a tab stack. Tab stacking is also more immediate in that you can just flick one tab on top of another, without changing window.
A big problem with Firefox's Panorama window is that thumbnails are very low resolution, and when clicking on a thumbnail, that thumbnail is maximised to full screen to become the web page depicted, but intermediate images are just the low resolution thumbnail magnified, which appears ugly, even disconcerting.
Opera's implementation is more polished; Firefox's functional but visually cruder.
Firefox 4 needs more polish: automatic sizing of windows in Panorama, and a smoother change from thumbnail to full screen browser page.