Last night, support for Fennec on AMO was pushed live. A big thank you to Mike Morgan and crew for making this happen. There are no recommended Fennec add-ons yet, so you need to search for them. Here are some results, based on the two add-ons I know support Fennec and are on AMO:
Extending this mobile browser is now fully operational!
The latest version of AVIM also supports Fennec, though it’ll probably be a few months before the new version gets approved.