Firefox addons are traditionally written using the same API the browser itself uses. That's what makes them much more powerful than Chrome extensions, but clearly more fragile to breakage.

They're starting to offer a separate extension SDK (formerly called Jetpack) that allows extensions to ignore the internal API and build against something more stable. Such add-ons can be installed without requiring a restart, and should be much easier to update, but many important extensions need more power than currently provided. (Ad block is a good example.)