Google's Chromium? That's slightly wrong - actually Google base their Chrome browser on Chromium, and Chromium is nothing to do with Google (I use Chromium browser, but I don't use Chrome by Google)
Chromium is a webkit browser -
Firefox 4 is available from http://www.mozilla.com/en-US/firefox/beta/
It runs well - I find it very quick and snappy on my linux box - though I'm pretty addicted to Chromium for day to day use (Firefox does come in very useful for tons of stuff).
You should definitely check out Opera 11 too, that's one I started testing last week, and it's also blisteringly fast (seems as fast as Chromium).
Many browsers now are taking cues from each other in terms of design, and the design of tabs in Chromium (putting the relevant information for the tab inside the tab) is going to take over as the standard for all browsers - it simply makes sense.
There are quite a few Firefox devs who think Webkit is the best option for their future - and despite Firefox coming ahead nicely, it's starting to feel sluggish next to Opera and Chromium. They're working on JägerMonkey and could well come up with something faster... so it's hard to predict where they'll go in the end.
The main thing I notice when doing a fresh install of Firefox is that (unlike Chromium or Opera) it needs to check it's add-ons - it is dependent on them for most of it's more interesting functions. With Chromium, simple tasks like using the mousewheel to change tabs is built in. With Opera, all kinds of mousegestures are built in, but with Firefox add-ons are required. I'd like to see this system improved first (and the whole 'restart' when you install add-ons is so outdated... crazy).
Also the demo http://www.mozilla.com/en-US/firefox/4.0b7/ants/ runs at 70fps in Chromium and only 24 fps in the new firefox so whilst it's still a great browser, it's not ready for the newer web content.