User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0) Gecko/20100101 Firefox/4.0 Build Identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0) Gecko/20100101 Firefox/4.0 userChrome.css file provides a powerful way of tailoring the Firefox's UI. Still, access to it is rather complicated. I would like to propose the introduction of "about:userchrome" as an easy way to edit this file. Reproducible: Always Steps to Reproduce: 1. enter "about:userchrome" Actual Results: invalid url Expected Results: open userChrome.css editor - basically, a big <textarea> with the content of the file + save/restart firefox buttons

Use this addon - http://webdesigns.ms11.net/chromeditp.html

ChromEdit Plus ====================================================================== > Use this addon - http://webdesigns.ms11.net/chromeditp.html uBlock Origin finds this in a 'Malware Domain List': ||ms11.net^ ChromEdit ====================================================================== <http://kb.mozillazine.org/ChromEdit> is outdated. It refers to <https://addons.mozilla.org/firefox/17>, which is no longer found. <https://addons.mozilla.org/search/?q=ChromEdit&sort=name&type=extension> not found at AMO. A long shot, I ran <http://downloads.mozdev.org/cdn/chromedit/chromedit.xpi> through the Extension Converter for SeaMonkey <http://addonconverter.fotokraina.com/> wondering whether the end result might be usable with Firefox 56.0.2. Unfortunately not. Aris-t2/CustomCSSforFx: Custom CSS tweaks for Firefox 57+ ====================================================================== <https://github.com/Aris-t2/CustomCSSforFx#user-content-instructions--howto--readme> about:userchrome ====================================================================== +1 Nice idea. Some other userChrome-oriented issues ====================================================================== bug 794533 - Use a more allowing license for userChrome-example.css and userContent-example.css bug 1404398 - userChrome-example.css is gone bug 1409065 - Reload userChrome.css without restarting (e.g. when opening a new window) bug 1410511 - userContent.css and userChrome.css are not consistently applied bug 1416044 - Add telemetry for userChrome.css usage A future without userChrome.css? ====================================================================== <https://blog.mozilla.org/addons/2017/08/10/upcoming-changes-compatibility/#comment-224171> draws attention to <https://bugzilla.mozilla.org/show_bug.cgi?id=332529#c14> under 'Run userChrome.js at startup (resp. when loading the main window)' (RESOLVED WONTFIX): > … (userchrome.css should probably go away for the same reasons, but that will be a different bug) – happily, as far as I can tell, no such bug at this time.