The new AMO is FUBAR. From an extension’s page, there is no quick way to add the extension as a favorite, or add it to one of your collections.
Also, the previous AMO wasn’t even working correctly. When trying to add an extension to a collection, the pop-up menu would incorrectly indicate if the extension was already in that collection, as well as erroneously indicate whether or not it was in your other collections.
If Mozilla is interested in building communities and a following, it seems like they would want to fix all these issues.