I'm not as familiar with the Firefox extensions, but in Chrome all extensions are turned off by default in incognito mode -- only those extensions that you've explicitly enabled will be active in incognito mode. Furthermore, the docs for Chrome explicitly encourage developers to have their extensions whether the browser is running in incognito mode, and adapt appropriately.[1] So it seems likely a few of the top extensions using the various storage mechanisms (localStorage or otherwise) are already doing the right thing; it would be interesting to see an analysis of this, however.

[1] http://code.google.com/chrome/extensions/trunk/overview.html...