And what is v1.80 based on? That does get confusing with all of these different forks.
In Firefox too. The GRE used in 1.8 is higher than in the K-meleon 74 branch just because Dorian is using the Extended Suppport Release (24ESR) but, actually, both are quite similar in up to date (or not up to date).
The main difference..., you'll have to test. 1.8 is more like 1.6 and K-meleon 74 is, maybe, just maybe a better aproach to the new Firefox GRE. I don't know exactly, but we can consider the 74 more official release than 1.8. But saying this, maybe I'm offending the group behind 1.8, that had its own work.
I won't try to give reasons. If you were using K-meleon in the past, the core of it, the browser you control, is still there in the same way. It just has been updated to the new GREs. Both 1.8 and K-meleon 74.