No need to repeat what’s already written and especially what’s up-to-date on the GitHub page. Just follow the instructions there. It’s worth mentioning that this is a client-only extension, so your data is protected!

You can find the project and follow the installation steps here: 🔗 https://github.com/webmonch/hide-my-mail-cloudflare

Note that in Firefox, extensions only run in temporary mode, so you need to reload them every time you start the browser.