See https://support.mozilla.org/en-US/questions/1426872

Thank you for getting back to me. I do not know what to do with the code(syntax?) you posted

Here are instructions for applying css:

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the css code, change the font size or family as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video: How to create a css file (Firefox or TB)

Test the method with a simple css that sets the background colour of the Folder Pane:

#folderPane {
background-color: lightgreen !important;}

Over my head. Thanks anyway

How do i make "Atkinson Hyperlegible Next" the default font for headings, toolbars, message list, etc?

Over my head. Thanks anyway

Then the answer is you can not.

Mozilla should consider making this font available in Firefox and Thunderbird. The Braille Institute has done much high-quality research in developing it. It is not the "prettiest" it is the most "legible".

https://www.brailleinstitute.org/freefont/