Splitting this out from bug 1504475 as I think it may be easier done in two parts.

I played around with this a little tonight, but I couldn't quite get it working. I'm attaching a test add-on that I was using, in theory it should turn message text red.

I also made a minor change to the toolkit schema, as an experiment to allow the content script to be specified for imap urls (I wasn't quite sure if we want imap or imap-message, but I used both) - I'll attach that patch next.

It seemed to load the add-on, but strangely the debugger was saying that the ext-contentScripts.js weren't being loaded. I'm not sure if that is to do with how our extension APIs are set up, or if there's something missing or not configured correctly somewhere.

There's other things I want to work on at the moment, so I'd be really happy if someone else took investigating this on. I was hoping it might be easy, but I suspect it just needs a bit of following through in the code to see what's not being fired.

This won't be necessary as a separate piece.