This is going to cause massive suck for Mozilla. See the URL for details. (I believe this to be an XP bug, but haven't actually checked on non-Intel platforms. Also, I believe it affects 5.0 and 5.1 as well.)
The libdl.so from glibc 2.0.x is not thread safe, nor it is designed to be made thread safe easily. At this point in time I can commit to assistance to the mozilla developers for getting this issue fixed, but I am unable to act as the main fixer. (yeah, a polite way of saying that I am taking patches). I can promise a glibc 2.0.x errata fix for redhat 5.2 once we get this issue solved.