It's not quite as crazy as it sounds; many of the most popular addons have a significant user-base on the nightly or beta channels (and now Aurora), so there's

It's not quite as crazy as it sounds; many of the most popular addons have a significant user-base on the nightly or beta channels (and now Aurora), so there's always been some pressure to update add-ons as changes were made to the underlying platform, even before those changes made it into an official release. Also, these days Mozilla has the "Add-On SDK" which provides an abstraction around the core Gecko and Firefox APIs that is more Chrome-like - more limited than what a traditional Firefox extension can do, but guaranteed to stay compatible for a long time into the future.