Taken from a mozillazine thread for extension development (see url). Prefpanes seem to have their bottom content truncated with 2.0. This wasn't an issue with 1.5, so I think this is a regression. I can confirm this for a mac and a pc, but the mac seems to have more truncated (compared to my pc). The forum thread includes a workaround.
Toolkit -> XUL Widgets seems more appropriate.
*** This bug has been marked as a duplicate of 283697 ***
(In reply to comment #2) I don't agree. That bug was regarding Firefox's preference windows, while this bug is regarding preference windows in general. That bug also indicated that there was no issue with the default theme, whereas this bug has the issue with the default theme. I'll hold off on reopening until I do a bit more research.