A password manager is an absolute security necessity. Having a standalone extension manage passwords makes it easy to sync them across multiple browsers and even to apps on your phone. Bitwarden is probably the best balance of easy-to-use and secure available.
Stylus is the best way to customize websites. There are some premade stylesheets you can find for popular sites, but if you're a little tech-savvy and understand CSS, this is a great tool to fix those little annoyances in websites.
This is the best adblocker by far. It blocks everything it can and it's very customizable. It works more efficiently in Firefox than in Chrome due to the later's adblocker limitations. I understand websites need ad revenue, but frankly they often make the web unusable due to covering everything and using so much of your computer's processing energy. It is very easy to disable the adblocker for sites that don't abuse ads, which I would encourage you to do.