Firefox 3.6.28 (.28 being the last version of the 3.6 series) seems to work fine on my system, one of the few things that does. ;)
It's the only browser I've used that lets me use the forum as intended, and zoom in so I can actually see the text. All the others seem to work 90%, but there's always some tiny little thing that doesn't work that annoys me, so I stick to 3.6.28.
But a lot of people don't like to use 3.6.28 because it's older and FF is up to version 18 now. I can understand that.