Why set up your browser?

A browser is a program installed on your computer through which you access the internet.

Set preferences

go to about > preferences

Set Firefox as your default browser
Fonts > Advanced - you can tell the browser to override fonts on sites with those of your choice, which can be helpful for disability reasons or if you want to insist everything is pink comic sans now
Home > choose firefox home content - for example, to prevent them recommending your articles (which will suck your time) or frequently
Search > Default search engine

Make your own using Firefox Color or browse the Themes for one you like

Go to burger menu > More tools > Customize toolbar