Is because my laptop only has 2GB of memory.
OK – it sounds worse then it is, but with normal use Firefox is simply way too heavy for my – not too shabby – Thinkpad T61 Core2 Duo T7250 @ 2GHz with 2GB RAM. Its not a stellar machine by today’s standard by its less then 3 years old and I don’t see why it shouldn’t be able to run a modern browser.
But with both Evolution (that requires a couple hundreds MB of memory) and Eclipse (at ~ 600 MB memory) I can’t also run Firefox which with just a few tabs open takes up close to 1GB of physical memory (and tons of virtual). My system just comes to a standstill, and lets not talk about running – oh, I don’t know – a terminal!
So I’m using Chromium and while its developer tools are not as good as Firebug, at least it doesn’t hog up all my RAM and with a few dozen tabs open my system is still pretty responsive.
It may be the process separation in Chromium that is better, allowing the operating system to swap out completely tabs that are not used. I’ve heard some talk about having the same thing for Firefox (project electrolysis – though at this point it seems to be focused on the Fennec mobile browser) and I do hope they get on with it because Firefox’s memory consumption has grown in leaps and bounds in the last couple of years and unfortunately my computer’s memory has not grown with it :-(.