You can try to press F12, go to network tab, open https://check.torproject.org/ and watch what info Firefox will show there.
But better method is to use “curl” program. Run this command: curl --socks5-hostname 127.0.0.1:9050 https://check.torproject.org/ -v and if result will be like this, then Tor proxy works for sure and problem is on Firefox side:
