If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should NEVER respond to such an alert to avoid getting infected with malware.

• only update Firefox via "Help -> About" or by downloading and installing Firefox from the Mozilla and never via a pop-up or link on a web page.

See also:

• https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update
• https://support.mozilla.org/en-US/kb/avoid-and-report-mozilla-tech-support-scams

You can do a malware scan just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Thank you! The second of the three links you provided included a link to the Official Firefox Download Page, and was surprised to see that it looked almost exactly like the full screen pop-up that prevented my streaming of MSNBC. The pop-up was missing the two menu bars on the top, the scroll bar on the right side, and the three links under the blue Firefox button did not work. Otherwise it looked identical. I had clicked on the blue button more than once and each time downloaded an installer / executable but I never ran it.

I have the free versions of SuperAntiSpyware, Malwarebytes, and AdBlockerPlus, none of which stopped this thing from happening. I ran a SuperAntiSpyware scan yesterday, so today ran a Malwarebytes scan and a Windows Defender scan, but neither one of them found anything. So I don’t understand what is going to prevent this from happening again. I also deleted what had been downloaded. Things are back to normal for the time being, so I’m glad about that. Thank you again.

FYI...using multiple A/V security software is fastest way to get infected as each will block the other. Also remember adblocker plus signed agreement with ads company to let ads through their software as well. Also never use Windows Admin account and that should be password protected and only used for updating - by using it you give permission for any software to install regardless of what security software you have installed. Instead create a Standard account that if any software installs will require Admin password and that will block unauthorized installs. But this is best left for a Windows forum to discuss this and for users to do their homework to protect the Admin from such stealthwares.