[inops]

userChrome.css support will be disabled by default in Firefox 69.[1][2] The user must toggle the "toolkit.legacyUserProfileCustomizations.stylesheet" preference to enable that.

Based on the name of that preference, as well as this developer page[3] saying "Support for the userChrome.css file and any of its elements described below are not guaranteed in future versions of Firefox", it seems likely userChrome.css will be dropped in the future.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1541233

[2] https://www.mozilla.org/en-US/firefox/69.0beta/releasenotes/

[3] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tu...

[wlesieutre]

That's a bummer, AFAIK it's still required in order to disable to top tab bar row for use with Tree Style Tab or similar.

https://hackernoon.com/firefox-hide-tabs-6e1f6cf63d95