First click on the Settings on the top right of the firefox screen, it should look like a gear. Once you click on that, click on General, which should also looked like a gear on the left side of the firefox screen. Finally, scroll down a bit until you see something that says Language and Appearance and under that should be Fonts and Colors. You can click on Fonts and choose the font that you want.

This does change the font of the webpages i visit, but not the menu bar. I am trying to change the font of stuff like the toolbar and bookmark bars but that option only changes the font of the pages I visit

To change the font in the Firefox user interface (menu bar and other toolbars) you would have to use code in userChrome.css.

• https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox

#navigator-toolbox { font-family:OpenDyslexic !important }

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• toolkit.legacyUserProfileCustomizations.stylesheets = true

• https://www.userchrome.org/firefox-changes-userchrome-css.html