I was raising similar concerns a few days back, but nice research into "whatever" the 24 ESR release branch means into Stuff.
Almost replied to your post along with Wraith's but I think you asked enough questions to keep things separated. So....what's ESR? Simple answer is ESR stands for Extended Support Release. Huge companies hate change, but software developers love it when huge companies use their software and ESR releases make a compromise that makes software more palatable to those huge companies.
When a version of a particular program becomes ESR, it's usually an older, established version (such as Firefox v24) that Mozilla has made a firm commitment that they will not add any new features to that version (thereby not introducing any new bugs), but will commit to back-code any bug or vulnerability fixes for a set amount of time (say, 18 months) while the regular software version moves forward to bigger and better things.
Now, how does this affect Pale Moon? Pale Moon was based on Firefox 24, an ESR release, so they were able to make all their coding changes and feature additions safe in the knowledge that if anyone on the Firefox side of things found/fixed a bug or vulnerability, that code would be shared with them. Since ESR releases are used by huge companies, there's a lot of scrutiny by non-Firefox & non-Pale Moon developers/employees meaning more eyes being more likely to find any problems that may exist.
Now, the extended support commitment for Firefox 24 has ended. Firefox's new ESR release is Firefox 31 and it will get the extended support and be used by the huge companies going forward, leaving Firefox 24 behind to be deprecated and forgotten. One may argue that Pale Moon has digressed enough from the original Firefox code by now that this doesn't matter & to a certain extent that is true, but the fact remains is that the Firefox 24 code-base is firmly, with certainty, and forever out of the spotlight being declared obsolete by its creators.
Going forward the Pale Moon team is on their own (which to my knowledge is only two or three people).
I think I'm slightly meta-concerned that Google, well, became "a verb" to replace "web search engine", but then they decided to make Android and are scaring Apple, then they decided "hey, let's do browsers" and A, made Chrome that lots of people like as is, then made *Mozilla* turn add-ons into Chrome Compatible things.
I'm not sure what to make of Google these days and this is coming from someone who has an Android phone & tablet sitting on his desk right now and wouldn't have it any other. Just yesterday my friend were reminiscing about the days when Google's mission statement was Do No Evil. When was the last time we heard anyone at Google utter those words?
So now that Pale Moon is going to decide to make their own engine, did someone "super-trap" them into that?!
No, no one was trapped, scammed, or anything like that, but for a while now Mozilla has tied the browser engine version to the browser version and that's really started to become limiting to Pale Moon. Something needed to be done so the Pale Moon browser could move forward on its own and that's what needed to happen: a browser engine of their own.
But so - if people can't just do old school "let's do FF forks", are we getting meta-played into something? So Mozilla is Doing Things, Pale Moon has been a serious leading contender spin, I'm using a 3rd tier one by Comodo called Ice Dragon, Vivaldi is trying to re-undo Opera-ish-things, and sky only knows what MS's Edge means into all of this!
Life is full of paths and those paths intersect and branch off in different directions. Spend all your time at an intersection straining your eyes to see what's down the end of all those paths & you'll end up going nowhere. I can see real benefits into moving to Chrome add-ons where there's a unified extension scene & not having to upgrade your extensions every time you upgrade your browser. However, I'm a little worried about all the telemetry and "suggested sites" that are just veiled advertising that's being added to Firefox. Fortunately, if you dig deep enough you can turn it all off, but it's a little troublesome the way things are heading.
I just want non-sleek old style menus where all 70 features are there, and people don't go "yay simple, so we can trick you into us hiding behind gear icons into cutting huge swaths of features!"
If you want your old Firefox UI back then check out the Classic Theme Restorer extension. It goes a long way to making things look sane again.
The Opera and Vivaldi developers just don't get it & I don't think they'll ever be more than niche products.