I don't know what "Rackspace" is, but my first question to you is to ask if you have ever made this work? Thunderbird gets signatures from either an internal store, within account settings, or from a nominated file, which for me has always meant one on the local filesystem. I've never tried it with an URL, URI or UNC type of connection. What mechanism do you anticipate using to load a signature from somewhere outside Thunderbird? I'm not saying that it can't be done, just that I've never heard of it being tried before.