This is looking more and more like a bug to me.
I created a new page, and used the code right from the demo page, complete with the Redmond theme. Still having this issue.
I'm guessing that something else on the demo page, unrelated to jqgrid (maybe the layout plugin?), is causing FireFox to rerender, so the bug does not show up there.
Can anyone else confirm this? Does anyone else have forcefit working correctly on thier site?