That is a Flash player.

Do you have problems with Flash on other sites?

Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Yes, I meant Flash player.

No, I do not have any trouble watching Youtube videos.

I tried the suggestions and even cleared all cookies and disabled all unnecessary plug-ins and extensions, but I'm still having the same problem. Should I uninstall and reinstall Firefox?