Frontend web developers are a small fraction of Firefox's users, and you can't expect them to tailor their processes and expend resources just so every niche JavaScript framework can trumpet its six-week release cycle.
For the same reason that you usually shouldn't use your distro's packaging system to install frequently changing development libraries, it makes more sense to just default to manual installation of developer extensions. You can always alert the developer if there's a new version. You may not even want the developer extensions to auto-update because then you have to include support for every previous version of your framework in each version, rather than having the opportunity to remove support for old functionality and have each extension version support only the equivalent framework version.