Palemoon - a FireFox fork that uses a different rendering engine and which strives to excise all the telemetry that Mozilla has in their product that reports back to them what you are doing with their browser. Also has deliberately kept the XUL addon's API which allows for better and more powerful integration at the risk of security (if you believe Mozilla) - trouble is that Mozilla have now disposed of their massive archive of addon's that use the old interface so unless the original developer has maintained their own repository for their addons they can be hard to find.
Also - because of other changes under the hood there are some internal differences that mean not everything is compatible. By the same token the new system (Webkit) Addons for FireFox just won't work in PaleMoon. Windows and Linux builds available and third party builds for Linux and MacOs at: https://www.palemoon.org/.
It might be worth noting the business models for Brave and Palemoon and certainly Wikipedia does mention interesting things about both browsers:
https://en.wikipedia.org/wiki/Brave_(web_browser)
https://en.wikipedia.org/wiki/Pale_Moon_(web_browser)