It'd be nice if we could use the correct terms for these things. I know that I'm probably picking nits, but there's a difference between extensions and plugins.<p>My simplified definition: plugins run native code, extensions run Javascript. Flash if a plugin, Firebug in an extension.
Still doesn't offer any <i>real</i> extension functionality; it's more a glorified, built-in version of Greasemonkey. This precludes such popular and useful Firefox extensions such as Firebug, Foxmarks, AdBlock, or just about any other extension besides Greasemonkey itself...