Hello Michael.

Although possibly not related to your problem, I have to remind you that the version of Firefox you are using at the moment has been discontinued and is no longer supported. On top of this, it has known unpatched bugs and security problems. I urge you to update to the latest version of Firefox, for maximum security, stability, performance and usability. You can get it for free, as always, at getfirefox.com.

As for your issue, which email service provider do you use, and have you tried contacting them for support? Also, does this also happen on other browsers or just Firefox?

Web mail using Firefox, or are you using an email program? Is that video embedded in the body of the email, or does it show as an attachment?