Privacy/security.
I second what [deleted] shared. In relation to Firefox use PrivacyTools as a foundation. To help select, add-ons, choose a search engine, and use privacy-related tweaks that suit your threat model. Keep up to date with this site, it is very informative.
Add-ons and tweaks that are related to aesthetic/customization.
Tabliss - A beautiful New Tab page with many customisable backgrounds and widgets that does not require any permissions.
Dark Reader - Dark mode for every website. Take care of your eyes, use dark theme for night and daily browsing.
This next add-on/Tweak I am sharing is essentially my all time favorite add-on/tweak when it comes to Firefox. Unfortunately this happened:
After upgrading to the 57+ version of Firefox, the Roomy Bookmarks Toolbar addon is no longer supported. This project consists in mitigating this non continuity of support.
So most of you have probably never heard of it until now. It's functionality was simple:
Roomy Bookmarks Toolbar saves space on the bookmarks bar. It's the only one of it's kind that hides the names of your bookmarks until you hover over them with your mouse.
But it lives on! Thanks to people a whole lot smarter than me.
Roomy Bookmarks Toolbar
Here is the link: Roomy Bookmarks Toolbar
And of course, mastering the basics....
15 Useful Firefox Keyboard Shortcuts