This stylesheet fixes tab groups in Zen. There are still some issues with it (see below), but I'll try to polish it furher when I find the time.
- Open
about:config(skip to step 5 if you have enabled both settings already) - Set
browser.tabs.groups.enabledtotrueif it isn't yet, by double-clicking it or tapping its ↹ button - Set
toolkit.legacyUserProfileCustomizations.stylesheetstotrueif it isn't yet - Restart Zen
- Open your profile folder (e.g. through
about:profilesand clicking "Open directory" next to the "Root directory" of the profile that is missing the "Open in new browser" button) - Place
userChrome.cssfrom below there (or if that file already exists, just append the contents below into it)