It sounds like your browser might be having trouble loading scripts or cached data from YouTube. Try clearing your cache and cookies first — go to Settings → Privacy & Security → Cookies and Site Data → Clear Data and restart Firefox. If that doesn’t help, open YouTube in a Private Window. If it works there, one of your extensions might be causing the issue. You can test this by starting Firefox in Troubleshoot Mode (under Help → Troubleshoot Mode) to see if YouTube loads normally. Also, make sure Enhanced Tracking Protection is set to Standard and that YouTube is not blocked under your site permissions.