It would be good to continue on to Firefox 52ESR, which should be doable. 52ESR is going to be the last Mozilla release for XP and may be maintained for longer or forked for XP (and Vista) users.
SeaMonkey is likely to die after 52ESR and is currently based on 52ESR with the next few versions planned to stay on 52ESR and there is a good chance that Mozilla52ESR will be forked to enable SeaMonkey to keep working with security fixes, there's rumours that some Linux dists are planning on maintaining it, which would include Firefox. Considering that we're always a year behind, we may be able to use the 52ESR code base for a few years
Thunderbird has also considered forking Mozilla-Central (Firefox head), but that seems unlikely right now and if they do fork it, it'll probably be 56 that is forked.
In summary, there is no great rush to move away from Mozilla and 52ESR may be viable for 3 or 4 years, by which time most browsers are probably going to be 64bit or in the case of QT based webkit, just as out of date as 52ESR. Chromium has the same problems as Mozilla with too fast of an update cycle.
All the other forks of Firefox I've looked at have been 64bit or dependent on working 3D graphics support.