Not sure if there will be an "official" one, but you can always manually install them.
I was wondering if there will be an "official" extensions list. In BT4 there were quite a few extensions to play with, and now with BT5 there are only two- NoScript and Tamper Data.
No adding, and tracking a boat load of extensions only adds more unnecessary work and overhead. It also adds bloat that some may not need to firefox.
These are easy enough for you to install on your own.