From the bug report , which also has a screen shot mock up "This would make it easier to identify the folders of diffrent mail accounts. This would also make sense for the top account nodes."
I have moved this to "discussion", because it was perceived to be a bug, not an idea.Based on Fjoerfoks' findings it would seem this is also not a bug.
Thanks all for reporting this. It's more "bug" than improvement, for which we have https://bugzilla.mozilla.org/show_bug.cgi?id=1889558. The action will be happening in that bug report and there's not really an idea to discuss or vote in, so I will...
