I've used opera a long time ago, was okay, but trying to get a semi-dynamic webpage to render on it the same as IE / NS was trying to say the least. Im running Firefox 1.5 atm, last time I tried 2.0 it crashed a few times, I regularly open 30 odd tabs (gotta love reporting spam e-mail) can't say I've noticed any slow down / memory issues with it even when running processor intensive processes even when I ran it with Memory intensive stuff not mich of a slow down..

Lurch - if you can't find it and your running 2.0 it's maybe not compatible or it's been discontinued although saying that a few extension I've got are hard to find (annoying really).

Maybe "Google It" rather than use the extensions search on the FF site

Done a bit more digging and it wasn't an addon, it was an about:config tweak.

Firefox Memory Cache

By default, Firefox does not use a fixed size memory cache - it uses a percentage of system memory. Setting a fixed size memory cache can often reduce memory usage - 4 or 8MB is sufficient in most instances. To set a fixed size memory cache, add the following pref using about:config:

browser.cache.memory.capacity

This is an integer pref, and can be set to the size you want the cache to be (in kilobytes).

Memory Usage Upon Minimize

There is a pref that enables trimming Firefox memory usage when you minimize all Firefox windows. This pref is for Windows only. To enable it, set the following pref to true using about:config:

config.trim_on_minimize

Couple more here. There's probably a few more obscure settings to play with if I was to look on Google a bit more.