[servercobra]

Is it closed to all types of new extensions? Or just certain kinds requiring certain permissions or in certain groups? I hadn't heard about this.

[dmix]

You can still install them on Chrome by downloading a directory and loading it via the extensions page (you have to keep the directory around though).

Firefox allows you to install .xpi packages directly, just by opening the xpi and clicking "install", after enabling the option to do so in about:debugging (unless you use a development version of Firefox then it's automatically supported). No directory/zips extraction required.