A simple add-on that does exactly as it describes.
Only reason for 4/5 instead of 5/5 is the lack of blacklisting websites. Although I would love all websites to be using Lexend, some just don't play nicely with the font and so I have to disable the add-on whilst using specific websites.

Lexend for Firefox is excellent!

It changes the font on the page quickly without having to reload.
The Lexend font really helps you read faster and reduces eye strain.