I recall a recent thread complaining that after the recent update, Fx was slow to load the first time. I can't find it just now, though, so I can't connect this back to that thread. But...
After the change in version, Firefox libs are no longer preloaded by readahead, because the directory listing in /etc/readahead.d/default.later is the wrong version. This is an interaction between the readahead RPM and the Fx RPM. The solution is to change references to /usr/lib/firefox-1.5.0.9 to /usr/lib/firefox-1.5.0.10 in default.later.
The real solution is to have readahead.d files packaged with the program RPMs (or at least have the firefox RPM update default.later), but I don't know the details of how that would work.