you can remove it without removing anything else
excuse the command line, but because it is universal,
and a good learning exercise, here goes
as root
rpm -e --nodeps firefox_name_ofprm.rpm
(the nodeps is very important, check the man page, man rpm for the excact syntax)
see my previous hint
konqueror / sub menu preference or option it is called then
change the settings in konqueror for files association type txt (then subtype html)
to point to new firefox version)
this add application, then make it top of the list
Hope it makes sense (it will once you have found the menu)