Build your own...........
You need the source as download from Mozilla and the spec file found here;
http://www.fedoranews.org/tchung/thunderbird/1.5/
Name : thunderbird
Version : 1.5.0.2
Release : 1%{?dist}
Use the locate command to find your local copy for the two additional files needed and copy them to your /usr/src/redhat/SOURCES location;
locate thunderbird.desktop
locate thunderbird.png
$ pwd
/usr/src/redhat/SOURCES
-rw-rw-r-- 1 root root 10212996 Apr 19 16:58 thunderbird-1.5.0.2.tar.gz
-rw-r--r-- 1 root root 216 Apr 19 17:04 thunderbird.desktop
-rw-r--r-- 1 root root 21835 Apr 19 17:05 thunderbird.png
Some minor editing of the spec file in your /use/src/redhat/SOURCES folder is all that is needed;
Name : thunderbird
Version : 1.5.0.2
Release : 1%{?dist}
Once everything is done and you are now ready to build it, from the /usr/src/redhat/SPECS location do;
rpmbuild -bb thunderbird.spec
The same hold true for firefox(slightly easier);
The spec file and related files can be found here;
http://www.fedoranews.org/tchung/firefox/1.5.0.1/
They go in the /usr/src/redhat/SOURCES location;
$ pwd
/usr/src/redhat/SOURCES
$ ls -l
-rw-rw-r-- 1 root root 8453058 Apr 16 11:24 firefox-1.5.0.2.tar.gz
-rw-rw-r-- 1 root root 230 Apr 3 11:55 firefox.desktop
-rw-rw-r-- 1 root root 20158 Apr 3 11:56 firefox.png
A minor editing of the firefox.spec file located in /usr/src/redhat/SPECS;
Name : firefox
Version : 1.5.0.2
Release : 1%{?dist}
Then build; rpmbuild -bb firefox.spec