Screwy Vista + Firefox Keyboard Character Problem

I’m just about read to rip out my hair here, and I can’t seem to figure out what the problem is: seemingly at random, Firefox will start to incorrectly interpret keyboard characters. The becomes a #. The / becomes an ѐ. When it first started happening, I thought it was Vista-wide problem, but I realized it wasn’t happening in Outlook, Windows Mail, Word, or anywhere else, including the Vista shell. Then I thought it might have been some sort of problem with the Logitech keyboard software, but it’s a Firefox-only problem from what I can see because if I shut down Firefox and re-start it, the problem goes away. In fact, now that I think about it, it seems that it only tends to happen if Firefox has been open all day or multiple days – which makes me wonder if it’s some sort of memory leak issue. Any other Firefox users seen a problem like this?