As I received no objections from the floor, starting with TenFourFox.next (whatever it is after 24), the 10.4 SDK will be the only supported target for linking and the leftover 10.5 SDK code still in the changesets will be gradually purged as it bitrots. Remember, this is only for linking against the SDK. The browser will still run on 10.5, and you can still build it on 10.5, but the only build target will be to make a 10.4-compatible browser. If you want to look at 10.5 specific code, you could look at Tobias' AuroraFox changesets, though quite a lot has changed since then.
If all goes well, you should have a testing beta of 24.0 at long last in a week or two, and then our localizing team will have a full cycle and then some to do translation work which should be hopefully plenty of time for Chris and our gracious volunteers. The plan will be to migrate everyone to 24.0.2 on 10 December as the new stable branch.