Firefox has also other type of extensions. Jetpack extensions ( https://jetpack.mozillalabs.com/ ) which don't require restarts. And there are also "mini themes", called Personas ( http://www.getpersonas.com/en-US/ ) which don't require restarts.

Firefox 4 is the first release to support restartless add-ons: https://developer.mozilla.org/en/Extensions/Bootstrapped_ext...

Add-ons written in the traditional way will still need a restart, until the authors adapt them to the new bootstrap API. Here are some add-ons that are restartless already:

https://addons.mozilla.org/firefox/tag/restartless

Not in anything I've seen, about 50 different extensions including context menus, blockers, all sorts I'd call major.