I've been using Gecko-based browsers for... well, more or less since Gecko became a viable alternative to its predecessor (whatever the rendering engine in Netscape Communicator was called). I sometimes use Chromium for sites which insist on using a webkit/blink-based browser but I never stick around them for the simple reason that sites look better on Gecko than they do on Webkit/Blink, mostly due to (in my opinion) better font rendering in Gecko.

Note that I say Gecko, not Firefox. While I currently use Firefox I keep on switching between it and Seamonkey, what used to be the Mozilla suite. I run nightly builds for both, when one of the misbehaves I switch to the other until I get fed up with whatever bothers me in that build and switch back.

Chromium often is a bit quicker, especially in javascript-heavy sites, but it it a memory hog. It also looks rough around the edges, the user interface is not as configurable (overly large tabs and fonts in the UI are a constant annoyance) and it's normal (Linux) builds are more unstable than Firefox and Seamonkey nightly builds.

Firefox and Seamonkey used to have another advantage over Chromium in that it was possible (and easy) to run your own sync server. Now that the 'old' sync engine has been deprecated for good this advantage is gone due to the lack of a self-hosted 'new' sync engine. This means I have to make-do without a sync engine as don't see why I would give so much of my data to any company.