My userchrome.css file to make Firefox Quantum look better with a fixed-size URL bar and changing URL bar color to indicate website security level. All scripts were found on the web. Credit to https://www.userchrome.org/ for script preventing size change.
To unlock custom CSS usage in Firefox 69 and newer: about:config > toolkit.legacyUserProfileCustomizations.stylesheets > true
To use this file, please create a folder called "chrome" in your Firefox profile and place a copy of the userchrome.css file in it, then restart Firefox. The changes can be undone by deleting or renaming the file and restarting Firefox.
Note: this file is not currently recommended for Firefox Proton (89+) because the tab rounding is excessive and the URL bar colors are tinged with gray.