I don't think this has been addressed elsewhere, but whenever there is a large amount of dialog in a comic with narration boxes, the narration boxes float instead of locking at the top of the comic frame when viewed in Firefox. DX said he gets that condition as well. Witness:
If you want to see any other examples, just click on any comic in the last 20 by non-donors.
I hear people have problems with slashdot tables & firefox and that the bug has been fixed, I haven't tried the nightly bug but it's possible the next version of firefox fixes this.