So there's an interesting parallel in Rust with clippy, which is a linter that uses internal compiler data structures to parse and find the things to lint. This makes clippy very powerful, but very, very prone to breakage. Eventually, clippy was simply upstreamed into the compiler proper, which meant any refactoring of the compiler had to fix all the clippy lint at the same time.

Linux is similar with drivers. Linux breaks internal kernel ABI all the time, but all the drivers upstreamed into the kernel are automatically fixed when this happens.

According to OP post, Firefox and addons developer would communicate when breaking changes happened, to make sure an upgrade path was doable. I wonder if a Linux-like system where a repo with all addons that developers could land refactors on when landing breaking changes would have changed the dynamic somewhat, by reducing the "addons developer tax".