the user.js file is a very underrated feature as well. i have a few computers and some of them are dual boot which means i have a lot of firefox installs so its great just being able to drop in the user.js file and have everything set up the way i like it
heres a few general ones. on my linux computers i have a bash script to install firefox, then it sets up symlink of the user.js file from my own config folder to the default .config folder. on windows i just manually add it
user_pref("browser.aboutConfig.showWarning", "false"); // disable about:config warning
user_pref("browser.startup.page", 3); // restore previous session
user_pref("browser.ctrlTab.sortByRecentlyUsed", "true"); // cycles tabs in recently used order
user_pref("signon.rememberSignons", "false"); // dont ask to save passwords
user_pref("browser.search.suggest.enabled", "false"); // disable address bar suggestions
user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", "true"); // enable custom css