Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0 20171016100113 Since Firefox 55, changes in userContent.css take effect when opening a new tab or window. It would be great if the same or equivalent was true for userChrome.css. There are many instances where it's inconvenient or impossible to restart the browser, like unfinished work or running downloads.

I just realized this is possible by editing userChrome.css with the Browser Toolbox's Style Editor. 1. Press F12. 2. Click the little cog icon in the top right of the developer toolbar. 3. Check both "Enable browser chrome and add-on debugging toolboxes" and "Enable remote debugging". You can close the developer toolbar afterwards. 4. Press Ctrl+Shift+Alt+I or Cmd+Opt+Shift+I to bring up the Browser Toolbox window. Confirm the security prompt. 5. Click Style Editor in the top toolbar. 6. In the list on the left, click userChrome.css. Edit it on the right and changes will be visible immediately. Click "Save" below the name in the list on the left to keep the changes. More details https://developer.mozilla.org/docs/Tools/Browser_Toolbox

(In reply to Gingerbread Man from comment #1) > changes will be visible immediately. Bug 1420713 filed because that's broken with layout.css.servo.chrome.enabled = true