Been using Chromium for 2 years now... Started using firefox beta versions more recently, and since I am planning to switch my desktop over to a ubuntu linux from windows (because I like the interface, and I already have another machine running windows 7 and a third running 8), I am still tossing around the idea of using the beta versions instead of Chromium as my main browser. But! I do like chromium's design better, and it does (seem) to run faster on my desktop, than the firefox beta versions (currently, anyway... both used to run really slow until I upgraded my ram to 4GB from 1)...
Also, I Haven't used any nightly releases, or aurora firefox either, because I'm too lazy to keep track of what I'm doing in 3+ browsers, although I might consider installing a stable version of normal firefox just for guests to use (I don't like people perusing my bookmarks, and so I keep my bookmarks in chromium, which I don't let guests use, and don't keep shortcuts anywhere for.)
It goes without saying, but I clear my history, cookies, download history, cache, etc. every time I close my browser. No need for people to pry into my life so easily. I also use a substituted IP address through a proxy server, but that's mainly for downloads and shit, so I don't get caught as easily.
In short, I think that I'll probably keep using Chromium, but I do like Firefox Betas.