The future of Firefox's add-ons arrived in Firefox 4 with the introduction of "restartless add-ons." Based on the new Jetpack API and Add-on SDK, restartless add-ons--also known as bootstrapped add-ons--don't require a restart to be used or removed. Not coincidentally, they also provide Firefox with a venue for competing directly with other browsers, which use add-on frameworks that were created after the technology that supports restartless add-ons was created.
Firefox doesn't offer many restartless add-ons just yet; at the time of writing, there were only 143. Also, there's been documented problems by existing add-on … Read more