Do you specifically want something based on Firefox? Or would you consider an chromium-based browser, as well?
Either or, reinstalled Firefox. There's really not much of a choice. Chromium is open source too?
Yes, chromium is open source. de-Googled chromium is also an excellent option to consider if you're not specifically looking for a privacy-centric browser. It doesn't collect information about you, but it doesn't have any proactive privacy features. But if you're not looking for a locked-down privacy browser, it's a good choice. https://github.com/ungoogled-software/ungoogled-chromium