Select all of the URL in the address bar with one click:
Find browser.urlbar.clickSelectsAll, right click, and chose toggle.
Firefox has this little built in feature that downloads pages from links that it thinks you may click on (eg. the top result on Google). This ofcourse means you use up a lot more bandwidth, and CPU power. To stop this, set network.prefetch-next to false.
To turn of chrome tooltips, you need to change browser.chrome.toolbar_tips to false
This next tweak is great for speeding up FireFox for broadband users.
find and change network.http.pipelining to true.
find and change network.http.proxy.pipelining to true
Change network.http.pipelining.maxrequests to 8.
Now instead of making one request each time, it will make 8. (this means pages are loading a lot faster!) Now a lot of places on the internet will tell you to set it to 30. Don't do that :) Firefox will normally only let you make 8 requests at a time, and even if you could make 30 requests, most servers wouldn't allow it, and result in your IP address being blocked.
To finish it off, right click, and make a new Integer. Name this nglayout.initialpaint.delay,and give it the value of 0. This tells the browser to wait 0 seconds before receiving information. Again, this speeds up the rate at which it loads pages.
If you have any more tweaks that you know of, please leave them in a comment, and I'll gladly add them to my list.