The add-ons manager regards 23.0a1 as "less than" 23.0 with no letter; however I suspect that at the moment, AMO accepts 23.0a1 (the current Firefox Nightly version) but rejects 23.0 (which won't exist until there is a 23.0 Beta, which is more than six weeks in the future). However I'm not sure whether AMO checks (or sets) the maxVersion in install.rdf, or whether these limits only apply to the rolldown in the AMO page where you declare to which application versions the add-on applies.