Bk52 (the browser, as test application) was initially built by upstream on top of a fork of the Mozilla ESR 52.6.0 platform code; the derived platform is named UXP.
Since then, both the original official Basilisk 52.9.x upstream project as well as roytam1's Serpent 52.9.0 fork have significantly diverged from that starting point, but have also diverged significantly between each other, too... IMHO, stating that Serpent 52 is "based on FF52 code" is no longer descriptive of the current situation...
Bk55 (the browser, as test application) was initially built by upstream (Moonchild Productions) on top of a forked Mozilla Nightly 53.0a1 snapshot platform code, with very few 54.x and 55.x code elements merged in; the platform that was produced was named Moebius; the app (browser) built on top of it was named Basilisk and was given an app version of 55.x.x, for (if you ask me) sensationalistic reasons ; much of the initial Moebius code, hence, can be described as a pre-53 Firefox snapshot...
Serpent 55 by roytam1 was forked off Bk55/Moebius; that upstream project was abandoned in favour of UXP (and Bk52/UXP); since then, St55 is being infrequently maintained as a code melting pot, merging bits of various other upstream projects (e.g. UXP, tenfourfox, Mozilla and even code from IceWeasel 53.x).
TL;DR: Current Serpent 55.0.0 has extremely limited affiliation, codewise, with stable Firefox 55.0