It might be. I tried using Firefox 1.5 last week, and experienced the same problem, and also had my cpu spiking up to 100% utilization. It's the first build - beta or not - that I've had a problem with. I went back to version 1.0.6, and the problems went away.
I'm not talking about the actual post width, I am referring to the frame on the left where names, post counts, avatars etc. are. They are wider for some than others, mostly people with a long Location, the text it not wrapping around, its just extending.
And yes, I am bored, but this actually pertains to the upkeep of the site