@DaveRo : the present thread is about using a mobile-friendly optimisation of the full-blown GNU/Linux desktop version of Firefox.
(Work that has been done as part of the Pine Phone community - several of the GNU/Linux distro available on it don’t even have Android App support - i.e. Anbox - to begin with)
This has nothing to do with the Android App version of Firefox.
This could be a solution in your case, given that it doesn’t rely on any Android support at all, and the desktop version’s Web Extension system is stable and mature enough to not rely on any whitelist.
But as pointed out by @michfu the main issue is that this application relies on much more recent version of gcc, wayland, etc. than what is currently available in Sailfish OS. Jolla is aware that it is seriously lagging behind on that point and are working on it. So maybe in some future version of Sailfish, this would be fixed too. (In the maintime, packaging local dependencies as suggested above seems a good possibility, too).
With regards to your question:
- the android API has substantially changed between 4.2 KitKat and 5.0 Lollipop. I suspect that Mozilla want to alleviate the extra burden of maintaining all the alternative fallbacks. (But would be losing in the process some users with older phones like you). As it is an opensource project, maybe some project will pop-up maintaining an alternate fork for older devices. (see how that hapenned back when mozilla dropped Firefox support fro Windows 95)
- the engine in Firefox 78 (GeckoView) has been quite overhauled and it hasn’t been extensively tested yet on Android: the whitelist is probably a measure to restrict to known good versions of addons that do not crash it. I am sure that there’s some setting in about:config that could be used to force non-whitelisted add-ons, I just don’t bothered because all those I need the most miraculously happen to all have been whitelisted.
They have more information on their blog. Over time more will be added, and eventually only the blacklist of known evil extensions will be maintained.