I've been using Firefox for ages now and have always loved it. My laptop is starting to get a little old now and starting to struggle with RAM. (I've upgraded to 2GB which is as good as it gets on this model) Firefox seems to use a bit more resources these days and does seem a bit slower and more buggy than what it used to be. I tried Minefield and while it's quite fast it's also quite buggy (it would be seeing as it's basically an Alpha release), I also tried Chrome from Google for about 3 seconds. Don't like it.
Finally I tried Opera 9. Uses next to no resources (realistically about 1/3 of what Firefox uses) and is blisteringly fast. For now I'll stick with Opera until Firefox improves.
The other thing that I liked about Opera is the little analog clock a la Vista that you can have as an add-on, very handy for an XP machine.