Don't call it a hunt. It's a single repository and single git pull command. You don't have to venture into the woods and hunt for it.

Right now we can't keep backward compatibility with your custom CSS, and it's too much work to test every possible config. That's why we have to keep it simple.

I see your point about not keeping compatibility with the .CSS file, but why is the .CSS file needed at all? Isn't it possible to integrate the handling of the different bars into the browser settings? I can customize the toolbar why not the placing of the bars?

Right now testers don't have resources to test it every release.

Perhaps they could change their minds if this post on Mozilla Connect gains more votes.