Because you did not replace the older firefox rpm package.
What I do when the newer firefox versions come out is;
1. Download the new source tarball (just like you)
2. Modify my firefox.spec file to reflect the newer version*
3. Build the new binary rpm file (rpmbuild -bb firefox.spec)
4. Install the new firefox-1.5.0.4-1.i386.rpm package
* I got the orginal firefox.spec file (and a couple of need icon files) from here;
http://www.fedoranews.org/tchung/firefox/1.5.0.1/
A little adjustment to the spec file may be all that is needed for SUSE, try downloading one of the older firefox source rpm packages to check and use as a spec file template.