If you aren't running the latest Fedora Core (version 5 at the time of writing), then trying to find and install the latest Firefox and Thunderbird (and to a lesser extent SeaMonkey, which does seem to have generic Fedora Core RPMs available) as RPMs is often extremely difficult. Even Fedora Core 4 - not in Fedora Legacy at the moment - is "beached" on the 1.0.X series of Firefox for no good reason.
Hence, I've written a "mozmkpkg" script which will attempt to discover the latest release of Firefox, Thunderbird and SeaMonkey (or you can specify a particular - older - version) and download the .tar.gz, construct an RPM out of it and optionally install the RPM. In the case of SeaMonkey, it "cheats" a bit and just downloads the generic pre-built RPMs I mentioned.
If you're in the en_GB locale and request to build a Thunderbird RPM, it will automatically fix bug #321742 by downloading an en_GB dictionary from openoffice.org and replacing the en_US dictionary in the RPM package, so yes, unlike the FC5 RPM from Fedora's own repositories, the Thunderbird en_GB RPM created by mozmkpkg will indeed be able to spell check with a British dictionary!
You need to run mozmkpkg as root I'm afraid (it uses the /usr/src/redhat build tree by default and, of course, may want to run "rpm" to install) - try "./mozmkpkg --help" for the command line options. If folks have problems with the script, please e-mail rkl at connect dot org dot uk and I'll have a look and try to fix them. You will also need lynx or links installed (either will do - use "yum install lynx" or "yum install links") because these are called to do downloads and directory parsing.
Current version in attachment - 1.00 as of May 2006. Released under a GPL version 2 licence.
The inspiration for this was Thomas Chung's Firefox/Thunderbird "build your own RPM" guide, which although it detailed the steps required was a) out-of-date and b) not automated. He's said to me that he now recommends that people upgrade to FC5 to get the latest packages rather than build their own RPMs, but my script is intended for those people who can't or won't upgrade for whatever reason, but still want to run the latest Firefox/SeaMonkey/Thunderbird. Heck, I'm still on FC4 myself!