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.
But if you earn over the threshold, you’re expected to pay taxes to the US government, no matter where you’re actually living or working, no matter if you’re also paying taxes wherever you currently are. The US is the only country in the world to assert that it has the right to tax its citizens remotely in this manner. It’s not normal.