I don't care how much memory Firefox uses, but I leave it open until it starts responding sluggishly (e.g. a second between hitting PgUp and the actual scrolling), which usually happens within 4 or so days.
Currently, I have 13 FF windows with between 2-10 tabs in each, and for me, this is close to my normal use. Windows reports that this uses about 750 MB of memory, and between 5-7% of a quad-code processor.
I can handle restarting FF once or twice a week, but it does mildly annoy me and I, too, would like to find a way to avoid it. I have tried to find a web page that causes the constant memory increase, but no site seems to magically fix the issues when I close it. I have also tried to isolate an extension/plugin, but also to no avail.
I use (extensions, then plugins):
- Adblock Plus
- Better Gmail 2
- Diccionario (Spanish Dictionary for spell-check)
- Firefox Sync
- Greasemonkey
- Java Console
- LogMeIn Remote Access
- QuickRestart
- Stylish
- Google Talk (and Video Accelerator)
- Java Deployment Toolkit
- Java
- LogMeIn
- Mozilla Default Plug-in
- QuickTime
- (Shockwave) Flash