Zen is a fork of Firefox, supports Firefox extensions, and retains the built-in access to the Mozilla extension/theme stores from vanilla Firefox. If you go to install an addon, it even gives a popup to "Add to Firefox".

It's a good browser with only minor issues. I'm on Win10; automatic updates don't always succeed, and it seems like it blocks some communication between the 1password desktop app and the browser extension because I have to sign into each separately. Otherwise, I prefer it to Firefox in pretty much every way.