OK. My fault....
This is caused by the Sun java plugin and by the Adobe pdf plugin.
Removing them allows firefox to startup and work.
Still get these AVCs:
Jan 10 21:39:31 fedora kernel: audit(1105421971.189:0): avc: denied { execmem } for pid=6696 comm=firefox-bin scontext=user_u:user_r:user_mozilla_t tcontext=user_u:user_r:user_mozilla_t tclass=process Jan 10 21:39:33 fedora kernel: audit(1105421973.479:0): avc: denied { execmem } for pid=6696 comm=firefox-bin scontext=user_u:user_r:user_mozilla_t tcontext=user_u:user_r:user_mozilla_t tclass=process Jan 10 21:39:33 fedora kernel: audit(1105421973.487:0): avc: denied { execmem } for pid=6696 comm=firefox-bin scontext=user_u:user_r:user_mozilla_t tcontext=user_u:user_r:user_mozilla_t tclass=process
tom