Just end the Firefox process in task manager and start it again. If this happens more than once every couple of months, then you might want to investigate the possibility of there being something else wrong with your system. Anything from bad RAM to a malware problem from previous IE use.
You might also want to look at any Firefox extensions you have installed. Have Firefox check for updates, and install any it finds. It also may be a good idea to hit the MozillaZine forums, and see if there are any known conflicts between extensions you may have installed. While rare, they do exist, and can be annoying.
In a sort of worst case scenario, you can always create a fresh profile. It's also rare, but if you've been using the same Firefox profile for several major versions, it's possible for corruption to occur. Especially if you used a pre-1.0 version to create the initial profile.
Finally, since you say it's a Compaq, which is among the bottom of the barrel quality wise (especially laptops), don't rule out any bundled software. Given HP's recent penchant for spying on reporters, I wouldn't put it past them having bundled software that contains some kind of malware. I would also keep in mind for the future to avoid HP/Compaq laptops, no matter how good a price they offer. If you have two similar devices, and one is significantly less than the other without any kind of rebate or other gimmick in place, that reduced price will almost certainly come at the cost of quality. This holds true for laptops, stereos, desktop computers, vacuum cleaners, washing machines, you name it. When it comes time to replace that laptop, I'd recommend limiting the search to Apple, IBM/Lenovo, and Dell which is in order of quality and price, high to low.