My main web-browser is Firefox. I was a user of Firefox Nightly but recently switched back to the stable branch. I use:

• On Windows: Firefox for almost everything, but if I need a a Chromium-based web-browser, I use Ungoogled-Chromium. And if I need more private web-browsing, I use Tor Browser.
• On macOS: Firefox, and sometimes Safari (for testing purposes).
• On GNU/Linux: Firefox and GNOME Web. I also have Ungoogled-Chromium installed as a Flatpak but I only use it to access pages that only support Chromium web-browsers.
• On Android: Firefox (Fenix) and Bromite.