I removed Windows 10 GUID from firefox.exe's manifest with Resource Hacker, then altered firefox.exe's last modified date by opening it and saving it again with Notepad++ for Windows to register new manifest (Resource Hacker doesn't do it). It's better to change the date with BulkFileChanger. Logging off might also be sufficient on recent Windows 10 builds. Now it thinks it runs on Windows 8.1 and behaves just like it does on said OS.