1) Consider adding Malwarebytes Anti-Malware or Emsisoft Emergency Kit. From my experience Avast is not the best in the AM department.
2) You should have a 2nd browser (Google Chrome, Opera etc.) as backup in case Firefox decided to go AWOL.
3) For extensions and plugins, consider adding: uBlock (blocks both ads and scripts, lightweight, no whitelist), Web of Trust (community rating for websites - treat it with a grain of salt), LastPass or KeePass (password manager), HTTPS Everywhere (forced encryption of connections - can break some contents, so be noted).
4) Always have a backup plan! Windows Backup, Macrium Reflect, Acronis True Image etc. a lot of choices out there. There are also cloud services if you are interested.
5) If you do visit phishing sites, do it in a sandboxed browser (Sandboxie) or virtual environment (Oracle VirtualBox) because phishing links can lead to malware as well.