The way we load glue code for extensions (dom-inspector, venkman, noscript, adblockplus) is invalidated
by this change, too.  We depend on extension_is_enabled being a synchronous call, but the new Addons
Manager has an asynchronous api.

http://www.oxymoronical.com/blog/2010/03/How-were-breaking-some-extensions-in-the-near-future

We may have to leave it up to users to load extension glue code themselves in their rc, instead of doing it
automatically.  Loading extension glue through our "conkeror.load.*" prefs no longer seems feasible.