I followed all the instructions but it doesn’t seem to work, I downloaded the following scripts from here https://github.com/xiaoxiaoflood/firefox-scripts:

• BeQuiet: https://raw.githubusercontent.com/xiaoxiaoflood/firefox-scripts/master/chrome/BeQuiet.uc.js
• Enter Selects: https://raw.githubusercontent.com/xiaoxiaoflood/firefox-scripts/master/chrome/enterSelects.uc.js
• rebuild_userChrome.uc.js

If you want a button to manage your scripts, including the ability to disable/enable scripts without needing to restart¹ Firefox or Thunderbird, save rebuild_userChrome.uc.js into chrome.