Geek to Live: Top Firefox 2 config tweaks - Lifehacker
Firefox homepage calls the web browser "fully customizable to your online life," and that's not just marketing claptrap. There's a lengthy set of advanced Firefox preferences that can customize the browser to your specific needs.
useful.

Tab close buttons
already done
-

Tab width before scrolling kicks in
To disable tab scrolling entirely, set the value to 0.
* Key: browser.tabs.tabMinWidth
* Default: 100
* Modified Value: 75 (fit in more tabs before overflow enables scroll)
* Alternate Modified Value: 0 (disable scroll entirely)
why is this not working for me?
--------------------------------

Fetch only what you click

Fx .6 and up: Firefox has this wacky little feature that downloads pages from links it thinks you may click on pages you view, like the top result on a page of Google results. This means you use up bandwidth and CPU cycles and store history for web pages you may not have ever viewed. Creepy, eh? To stop that madness, set the network.prefetch-next key to false.

• Key: network.prefetch-next
• Modified Value: false

sounds good. done.
-

Limit RAM usage

All versions: Goodness knows I've done a good amount of belly-aching about Firefox's voracious appetite for RAM. (It's consistently the most memory-intensive process on both my PC and Mac.) Happily a simple config tweak got Mem Use right back down to a more comfortable number. Along with the previous prefetch mod, set your browser.cache.disk.capacity browser.cache.memory.capacity to a value that fits your total RAM.

• Key: browser.cache.memory.capacity
• Modified Value: Depends on your system's total memory. According to Computerworld:
  

  For RAM sizes between 512BM and 1GB, start with 15000. For RAM sizes between 128MB and 512M, try 5000.

sounds good. done.

--------------------------------
keep in mind as option:
Session restore
As a blogger and web mail user, it breaks my heart to recount how many times I've composed a long post or email message, then accidentally closed the tab or browser and lost all my work. No more! With Firefox 2, set the browser.startup.page key to 3 to restore your browsing session - with form entries intact! - every time you start your browser or undo close tab after a wayward click.
Note: By default, Firefox 2 automatically restores your session if your browser crashes - but this does it every time you restart your browser normally.
* Key: browser.startup.page
* Modified Value: 3
* Default: 1 (open your specified homepages)
Update: Ryan points out that doing it this way is like swatting a fly with a sledgehammer. To do it the normal person way, from the Tools menu choose Options, and in the Main area, select "Show my windows and tabs from last time" from the "When Firefox starts" dropdown. D'oh!