A great news for the users who hate restarting Firefox after installing (or uninstalling) add-ons to apply changes: Restartless Add-ons are now available in Add-on gallery. The restartless feature is only for the users of Firefox 4 (currently in beta).
Restarting Firefox after installing add-ons is one of the most annoying things about Firefox, specially when you love to discover useful add-ons. Chrome does not require such restart and users can easily install or uninstall extensions.
Mozilla has started automatically tagging restartless add-ons with tag "Jetpack" so that the users can find such add-ons easily. You can follow this link for "Jetpack" add-ons. Previously, Jetpacks were available separately in Jetpack gallery, but now have been merged in AMO with the conventional add-ons.
Moreover, don’t expect that all add-ons will not require restart in future. Actually, the restartless concept applies only to those add-ons which have been developed using Mozilla Add-on SDK (Jetpack SDK / platform). Mozilla says, "As the SDK is still in alpha, not all of the add-ons in this tag will work with all beta versions of Firefox 4. We expect compatibility to increase dramatically when the Release Candidates ship.".