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I think that you have a browser plugin installed which is interfering with CiviCRM. 

For example, this page suggests that it may be a Firefox addon called "YouTube Auto HD". 

Troubleshooting

For troubleshooting purposes, I'd suggest trying each of these to gain more info:


Try with a different web browser (i.e. Chrome vs Firefox).
Try the same browser, but with all extensions/plugins/addons disabled.
Try clearing your browser cache.


Diagnosing

Hopefully the above steps allow you to learn more about the root cause of the problem. So if disabling all your browser plugins did the trick then you could try disabling them one-by-one to find the culprit. 

Fixing

Then if you do find that one plugin is giving you trouble with CiviCRM and keeping that plugin installed is important, then I'd suggest either:


contacting the plugin author to complain about its behavior
finding a suitable replacement plugin that doesn't have the problem


In theory you could submit a bug report to CiviCRM, but given the behavior you've described, I think it's unlikely that we'd be able to easily change CiviCRM to make it compatible with a browser plugin like this.