The titles in the collapsible fieldsets are missing!

The titles of collapsible fieldsets in the forms won't show up correctly with mofilla firefox 2 (I'm not sure about previous versions).
The titles of the fieldsets are missing. The strange thing is that when I start firebug the missing titles pop up! When firebug is on and I refresh the page the titles disappear again.. This behaviour doesn't occur with fieldsets that are not collapsible.

IE shows titles normally.

Tech is awesome theme and I would like to use it but this bug is pretty annoying..

Any suggestions how to find out why the titles in the collapsible fieldsets disappear in Firefox??

Thanks in advance!!