While this is extremely dubious behavior from Mozilla, and reminds me why I stopped donating to them (the moment Firefox started requiring _their_ signature in order to load addons), Mozilla still has the "automated signing API" in place. Supposedly, this API allows to get an XPI signed as long as it passes a series of automated checks. So it's worth a try.
This was the excuse they used anyway when trying to justify their signature requirements were "not a walled garden". I didn't believe it of course.
You can also just mark the addon as "not listed in AMO" when submitting it to addons.mozilla.org and it will not be listed on the store, but it will be signed. More details in https://developer.mozilla.org/docs/Mozilla/Add-ons/Distribut...