I have been using Ubuntu Operating System for about 2 and half years now. If you are using Windows 98 or older, you should consider a newer OS. Ubuntu is a Linux based OS and is the most popular, at least for now. Even if you are using XP, you should think about a new OS. Most Linux distributions have more up to date drivers, features and software programs. Also it is great on-line, worrying about malware and viruses is minimal. Using Firefox on a Linux OS is no different than on Windows.
The biggest draw-back is what to do for Windows programs. There is a decent windows emulator called Wine, in which many programs will run. My favorite Windows program is Ancestral Quest, a genealogy program. Unfortunately, I can only view the people and families on AQ. If I make any changes or click on any web links it crashes when I exit the program.
I haven't quite given up my XP machine. I use two machines with a KVM (Keyboard, Video, Mouse) switch. It allows me to jump back and forth between computers. Its especially good for surfing genealogy sites on my Ubuntu computer and jumping to my XP to compare family info. Listening to music on one and downloading on the other. I can do true multitasking without lagging down a single processor.