This has been indeed asked multiple times here , the most recent one was just six pages back in this very thread... :
https://msfn.org/board/topic/182647-my-browser-builds-part-3/?do=findComment&comment=1199196
Answered previously by roytam1 on Feb 8th:
https://msfn.org/board/topic/180462-my-browser-builds-part-2/?do=findComment&comment=1195399
My own answer from back in February:
https://msfn.org/board/topic/180462-my-browser-builds-part-2/?do=findComment&comment=1195384
(NB: Not meant to offend anyone ... )
TL:DR: appVersion numbers simply don't tell the whole story and people should not just stick to them...
Latest NM 28.10.3a1 has all the code found inside unstable official Pale Moon 29.3.0a1, minus portions of the official code it was decided we shouldn't adopt, plus code we have kept but upstream have gotten rid of...
And since you're anxious about Web Components implementation, upstream haven't made any leaps towards that goal, there's a dedicated issue in their tracker
https://repo.palemoon.org/MoonchildProductions/UXP/issues/1361
coupled with
https://repo.palemoon.org/MoonchildProductions/UXP/issues/1344
where you can monitor their progress (or lack of... ); until recently bounty material, if/when something is added there, be sure it'll find its way into New Moon, too, irrespective of its advertised version number...
Is it clear for you now?
Best wishes!