How would you do that latter part? I currently have a userchrome.css for something similar but I wasn't aware that you could have conditionals in css.
The first result on Google (https://gist.github.com/olmstadfm/d6b4d37219e957d9cdcdb3ec88...) also seems to employ the :hover selector to accomplish hiding the tabs. I don't know how effective this particular script is, but it makes sense to use such a system.
Brilliant! Thank you!
https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snipp...
https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...
Tabs like History and Bookmarks won't hide but TST does.
#main-window[titlepreface*="Sidebery"] #TabsToolbar {
visibility: collapse !important;
}