The Firefox 3 status meeting minutes have an interesting note. One of the items discuses what extensions should be included in the final release of Firefox 3 as part of the core browser. The list includes numerous extensions such as Firebug, TableTools, and Searchbar Autosizer (note that the full list of nominated extensions is at the end of this post).
I am not a heavy add-on user as I believe that, by default, Firefox provides almost everything needed to browse the web with. Outside of a few extensions, such as the BlueOrganizer and Scrapbook extensions, most of the extensions I use are small add-ons that provide for an increase in browsing efficiency and speed. This is the type of extensions that should be folded into Firefox. Those extensions that have little impact on users who choose not to use them but which provide increased productivity for those who do.