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.)

As is vimperator and pentadactyl.

In short: Not with the old API. In this recent newsgroup post, you can see a list of the things that could break an extension, and it's a lot.

http://groups.google.com/group/mozilla.dev.planning/browse_t...

The work to solve that is mentioned in the thread.

AFAIK, Chrome plugins still have much less access and power than Firefox plugins, which makes it easier for Chrome.