Rated 5 out of 5 stars

This fixed most of the issues I had in migrating to v78, including reducing space between lines in the message summary pane.

I wasted a lot of time trying to use the userChrome.css file to suppress the envelope icons next to the messages in the same pane (as described above)... eventually I discovered that I needed to also go to Options-General-Config.editor and toggle
toolkit.legacyUserProfileCustomisations.stylesheets
to "true"

Léirmheas ar leagan níos sine den bhreiseán seo (3.0b5).