There are technical problems that are being identified and solved to general availability of addons on Firefox for Android.
Some problems stem from Android limitations. Running addons in the main process can ruin its responsiveness, and I think there was some study that showed people tolerate that much less on smartphones than computers. On the other hand, running them in an extension process that Android may kill at any time can cause addons to malfunction as they aren’t ready for that (and that’s something Manifest v3 is going to help with). In both cases users would blame Firefox, not their addons, because the cause of problems would be unclear.
Some problems are caused by many phones having metered internet connection with reachable caps. Updating the malicious extension stoplist regularly seems to be pretty fine on a desktop, but you likely won’t like Firefox “wasting” your mobile data to ensure your extensions aren’t mining the cryptocurrency-of-the-day on your phone, stealing your passwords or personal data or doing other nefarious things.
A number of APIs and features not used by the current set of mobile extensions may be not available yet even though one could reasonably expect them. For example, the browser.tabs API expects the environment to know the full list of tabs, and that’s not the case on Android. Extensions have no access to bookmarks on Android either. In many cases that’s caused by essential data being kept in the Android app itself instead of the Gecko engine embedded in it.
I do understand that we’ve been living without arbitrary Android extensions for years already and that feels really long, but I’m pretty sure Mozilla is working on making them happen and is going to allow them sooner or later.