In the coming months Mozilla will launch support for an open ecosystem of extensions on Firefox for Android on addons.mozilla.org (AMO). We’ll announce a definite ...
I don't know about pre-79, but their current version supports a very, very limited selection of extensions, many of which are to specifically improve the mobile version of Firefox. Currently, only a total of 22 extensions are supported, many of which share the same purposes.
Is still does, experimentally, if you enable developer settings, rather unintuitively through a Firefox Add-Ons account. Developer settings are not available in the official release but the Nightly builds as well as some forks, like 🦊Fennec, include them.