Firefox 4 was released ages ago but I've been waiting until my PPA updated it and I had it installed on my Linux system properly to review it. Overall I'd say my first impressions are - wow.
The very first thing that jumps out at you is the new look of FF4; it sort of looks like a hybrid between the newer versions of Internet Explorer and Chromium.
I've not really read any reviews of FF4 as of yet so I'm largely unsure what the community thinks of the new UI – I like it. I've never been a massive fan of either Chromium or Chrome's UI but there's something about this I like. Moving the tabs up to the top seemed like a bold move and ones that sits well with me.
The only other things I was immediately concerned about were the Home, Refresh and Stop buttons. They've been shifted too, the Home button now resides to the right of the browser; another strange move but another I'm oddly comfortable with whilst the Refresh button and the Stop buttons have been combined into one located at the end of the URL bar. Combining the two buttons is, in my humble opinion, sheer genius.
However. I am smart enough to admit that this whole “tabs on top” thing is a fad – and one that will be quick to fade with the tablet market once that goes. The whole idea of moving tabs on top and condensing the interface is to save on vertical space to squeeze a fe extra pixels out of the pane that actually views the web. This is designed primarily for netbooks and tablets. Netbooks are already in decline and Tablets are probably going to follow suit – Apple's iPad might keep the market going for a while but the App Store™ (See what I did there?) refuses to stock Firefox or Chrome.
Speaking of app stores. The Firefox Add-Ons screen now opens up in its own tab; app store style. This I'm not too fussed about as I didn't particularly see any great need for it but I'm definitely not complaining.
In terms of functionality – I've noticed a significant speed increase when both loading the program and with rendering pages. I've got a version of Chromium here (Not sure which, whatever's in the Ubuntu repos) and I can't discern a difference in loading web pages from sight alone.
All-in-all; Excellent work Mozilla. Keep it up.