JetPack, as we've previously summed it up, falls somewhere between the site-specific scripts of GreaseMonkey and full-blown Firefox add-ons. They can add to Firefox's chrome and bring up dialogs if you've got JetPack installed, but they don't require a total browser restart or compatibility check to install — just a site refresh.
At the gallery, you can install JetPack and try out some neat stuff, like the GTranslatifier, which can translate entire pages or selected text with a status bar click. There's also a JetWave notifier for Google Wave messages and a sidebar tweak, Thumbtabs, that offers Opera-like visual browser tab thumbnails in a pop-out sidebar.
Found a favourite JetPack to share? We'd love to see the link in the comments.