I use several extensions in Iceraven — Privacy Redirect; Bypass Paywalls Clean; I don't care about cookies — that work usefully. If there are any missing APIs, I haven't noticed them. These extensions can't be installed in Firefox for Android.
There's no technical difference between Iceraven and Firefox, except that Iceraven has a more generous allow-list. This isn't a technical restriction; it's a choice by Mozilla.
It's a valid choice — if they're aiming to curate a set of high-quality extensions, and prevent the use of lower-quality extensions — but it's a choice nonetheless.
Mozilla is also looking at monetizing their extension ecosystem by allowing the top makers of extensions to pay for placement. Perhaps, Mozilla could start letting only the highest paid extensions on Android.
Would this be "valid"? Uh . . . sure. It is also blatantly user hostile. I would rather Mozilla allow me to choose what I run, but Mozilla definitely isn't going to give the users that sort of freedom on release builds.
So a few extensions have been grudgingly permitted with extra scrutiny given during review, but they don't want to extend that effort to all add-ons, they don't want to back down to accepting the previous level of security, either, Android won't change its handling of child processes (and even if – any change there would take years to percolate throughout the phone ecosystem, plus a number of OEMs somewhat infamously are even more aggressive about killing seemingly unused processes), and re-architecting add-ons to cope with randomly being killed isn't an easy and immediate solution either.