Since Firefox is open source, instead of just waiting for Mozilla engineers to get around to it, couldn't someone just open a pull request?
In practice, this only works if you provide the password manager that will be chosen once and for all.
zaarn's suggestion "providing an interface for password managers" is an interesting alternative.
And I haven't looked but I imagine changing the encryption algo isn't a huge task, I wasn't suggesting that a non-Mozilla worker implement something huge like zaarn's suggestion, which fwiw I think is awesome as I don't use FF built-in password manager and find it just gets in the way.