You can safely install Thunderbird without the Orbit dep. Thunderbird is merely the precompiled package from Mozilla, packaged for easy removal. If we built it ourselves, we couldn't call it Thunderbird and use the official images. Orbit would be listed as a dep for Firefox as well, but my package script removes the dep from the slack-required. I didn't notice that it showed up in Thunderbird as well, so I didn't script for it.