The irony doesn't escape me that if they'd just turn off the extension restriction there are like a thousand extensions that would do that and more for any audience where such a thing still matters
I haven't tried to build Firefox for Android in a really long time but I wonder what the level of effort would be to just track the release tags with such a patch applied (i.e. the world's shallowest fork)
Negative, I just tried https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/ (which, delightfully, wasn't 0.99.beta-patch3 or some such nonsense) but it for sure was not "allow me to install whatever add-on I want". It's the same stupid list as normal Firefox for Android
Maybe you're thinking of Firefox Nightly, which I do run from the Play store and is better than "normal people Firefox" but this for sure is not the good old days of "Let Me Install Violentmonkey not stupid Tapermonkey"
Weird, I thought Fennec had those. I downloaded it and use it for browsing, but I mostly don't browse on my phone. From a web search though, I found out you can go into settings, click on "About Fennec" and click on the logo five times to unlock debug mode which apparently gives you access to custom add-on collections. You have to go to the firefox add-ons page and make a custom collection under your account for it to be useful. This presumably allows all add-ons through this method.
