Comment 6 for bug 922264

This isn't a Thunderbird bug then :)

Thunderbird, like most applications, uses gio to look up protocol handlers. In 11.04, this lookup was delegated to gvfs which looked up the handlers from some keys in gconf. In 11.10, glib was changed so that gio uses the same mechanism that already exists for looking up mimetype handlers (eg, desktop files), using the x-scheme-handler types for protocol handlers.

It looks like your configuration tool hasn't been updated for the glib changes, else there would be a "x-scheme-handler/http=chromium-browser.desktop" in the "[Default Applications]" section of your mimeapps.list