Firefox isn't "the fastest browser in the world" on every single computer that uses it, although some members here would lead you to believe this.
If you have tweaked it as much as you want to/can, and it's still considerably more slow than your current browser, then don't use it.
I will tell you though that there isn't a "set" incriment/variable for each of the fields Astroman listed. You have to try different numbers until you get it to run better with your connection. And what Astroman posted is about 1/10th of what I have done to increase the speed in Firefox, and i've probably only done about 1/100th of what I've seen online lol. So don't give up if that's the only thing you've tried.
After all my tweaking, Firefox is very fast...but Opera is still faster with no tweaking at all. I use Firefox still because it blocks EVERY pop-up and banner on webpages. I can't even see the ads on this site anymore since a few days ago when I got a new extension to block ads. Very helpful. That's jsut my computer though.
See? No ads

Blocks almost everything on ad-ridden websites. Which in turn lets the actual site load faster.