Looking at the recent checkins to the Firefox source tree, you can see that Ben Goodger has checked in the initial files for the upcoming Extension Manager (see bug 170006). The Extension Manager aims to provide for extension uninstallation (currently, you can only disable via the UI - you can, however, remove extensions manually), extension updating, and (hopefully) some level of standardization of extension installation UI screens.
Forcing extensions to have to provide a choice of whether to be installed to the profile folder or to the application folder is another thing I'd like to see (or at least warn the user that the extension will be installed to the profile folder). Probably wouldn't matter too much when you can easily uninstall extensions with the Extension Manager.
Henrik Gemal reports on the same thing, and points out ASCII art mock screens and Ben Goodger's writeup on the new take on extensions. Mozilla.org also has a section on Application extensions, where the Extension Manager is described (some pages are empty at the moment). Interesting reading all.
There are also plans for a Web Front End (see Ben's new take on extensions), if you were wondering how updating extensions was to be done. The Web Front End will also be the central link and information repository for compatible and verified extensions (much like what Extension Room is now) - a partial solution to the malicious XPIs problem perhaps?