Please do NOT install that suggested MediaPlayer. This message is caused by an extension I recommend removing, using the following steps:

Please try disabling ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Cmd+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then case a critical eye over the list on the right and, if in doubt, disable (or remove). Especially if you find something called MediaPlayer. Bear in mind that all extensions are optional and none are included with Firefox when you first get it.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that get rid of the messages?

http://www.clamxav.com/ free malware scanner for Mac OS X https://discussions.apple.com/docs/DOC-3291

You can also try https://www.malwarebytes.org/antimalware/mac/index.html