It was explained here why jumping to the latest-and-greatest (while it sounds like a good idea) is not a good idea after all.
I also picked some quotes from the blog that flypig has.
'As a result, to get through a full build it generally needs to be cancelled and restarted multiple times. Gecko supports incremental builds, so it’s not as painful as it would be otherwise, but it still makes it hard to determine exactly how long a full build takes. In my estimation it’s between five and eight hours on my development machine (14 cores, 16 threads, 32 GiB RAM).
Upgrading gecko for Sailfish OS is a big job. Just checking the git history I can see that the upgrade from ESR 68 to ESR 78 took in the region of nine months work from a team of contributors. Like I said, it’s a big job.