So now it's time for you to make your first build (and, if you feel adventurous, find a problem and try to fix it, but let's take baby steps). Officially, we have documentation for that already using MacPorts. A semi-frozen build of MacPorts what I use on my G5: I have three trees, one being the main testing debug tree which pulls from Github, and then two local subtrees that pull from the local debug tree (created with git clone --shared so that they are about 25% the size) which I use to make rolling G5-optimized (for my Quad) and 7450-optimized (for my iMac and iBook) builds. I do my work in the debug tree and make sure everything functions properly, then check it in and git pull and gmake -f client.mk build in the optimized subtrees to roll up the changes. When the subtrees are happy too, I'll git push from the main debug tree into Github. I consider this as officially supported a solution as presently exists under the circumstances. The Quad runs TenFourFox directly from the G5 subtree now.
However, MacPorts does have a lot of prereqs and requires some additional prep time (sometimes many hours) to build the tools from source. Macintosh Garden has an "unofficial TenFourFox toolkit" that contains an Automator workflow, a supervising script and a fully precompiled toolchain. You will have to install Xcode first (2.5 for Tiger, 3.1.4 for Leopard), but that is the only apparent requirement, and multiple users have reported it builds the browser successfully.
One common problem that gets reported on non-G5 systems is the dreaded internal compiler error. However, when the build is restarted, it usually progresses and continues for awhile without incident. The problem is likely tied to memory pressure and compilers really thrash memory. If your system hits this a lot and starts to annoy you, consider removing -j2 out of the build flags in whatever .mozconfig you're using (change your copy in .mozconfig, not the master *.mozcfg). This will only run one compiler instance at a time, which is slower, but requires less memory and is more likely to complete the build in one shot without manual intervention.
If you really don't want to build it yourself, however, you do have at least one option: InterWebPPC. This is a modified build of TenFourFox that explicitly removes some features for performance, so it is not equivalent with TenFourFox, and it is not necessarily built on any particular schedule either. It also does not have separate G4/7400 and G4/7450 builds, though this may not be noticeable on your particular system. You can download prebuilt binaries for G3, G4 or G5 as well as compile it from source using the "unofficial toolkit" above. I haven't seen other downstream builds yet but if you know of one, plan to make one or are using one, post it in the comments.
There are a couple other security fixes I'm reviewing, and I'm toying with some Github specific hacks to deal with its dependence on async/await, but these again will not be done on any particular timetable (I'll post here when or if I get around to them). Still, some of you have already built the browser successfully, and if you can build TenFourFox on your Power Mac you can build pretty much anything. Perhaps this might spark some additional development interest ...