Only available for Firefox
To install: Enter about:debugging in your search bar. Click "This Firefox" on the left-hand side, then "Load Temporary Add-on". Select the manifest.json file, upload it, and enjoy your new web-browsing experience.
Note that add-ons don't work in the about:debugging page, so you'll have to navigate somewhere else to see the magic.
Official instructions here: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension#trying_it_out