Firefox is fully compatible with Windows 10. I use Windows 10, Linux, and Android and I run Firefox on all 3 different operating systems.

Are you using the latest version and are you downloading it from Mozilla's official site and not one of these 3rd party download sites? If you aren't certain that you did, I would reinstall it with the latest version from Mozilla. Don't use a 3rd party site even if they are above Mozilla on Google search because many of them either have viruses or adware or want you to download other random software that you are unlikely to want or need because that's how they make money and their garbage bloatware could potentially be slowing your system down.