For anyone reading this in the future, what worked for me is downloading the .xpi file into its own folder, renaming the .xpi file to .zip, opening and extracting it in that same folder, then opening Brave or Firefox extensions page, put it in developer mode temporarily (upper right corner), then click on “Load Unpacked” in the upper left corner, point it to the folder you made, then click okay or ‘select’. That’s it. Done.
Thank you Bikkuri for the tip on changing the .xpi file name to .zip. That made the difference plus the rest of the instructions.