Even on Nightly, the procedure to install non-whitelisted-by-default add-ons is somewhat arcane though, and AFAIK even after following it you can only install the current version of add-ons that have been officially published on AMO, i.e. no installing a different add-on version or manually installing an XPI (not even a signed one).

Also lots of other little papercuts: https://news.ycombinator.com/item?id=26470454