Could be. I’m running XP 2002 SP 2. I’ve only encountered a few FF-incompatible sites, most notably Turbo Tax. I have to use IE to get my tax returns to print and file electronically.
All of my MS office apps run without a hitch. It’s my work computer and only the Director of IT and I use Firefox. He didn’t start until one day he was rooting around on my box and saw that I was using it. He tried it and loved it.
I just went to the TurboTax site, works just fine for me, and I’m not using IE.