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

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 https://bugzilla.mozilla.org/show_bug.cgi?id=1541233 [2] https://www.mozilla.org/en-US/firefox/69.0beta/releasenotes/ https://www.mozilla.org/en-US/firefox/69.0beta/releasenotes/ [3] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Modifying_the_Default_Skin https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tu...