In the same player? That is a website issue.

Have you been using an ad blocking extension such as Adblock Plus or uBlock Origin? As sites evolve, they sometimes lose effectiveness and need an update. Or sometimes an update breaks something.

In addition to the above - in case you are using uBlock Origin, see :

https://github.com/gorhill/uBlock/releases/tag/1.13.10

Hi, thanks everyone. I've used Firefox and Windows 10 for years. I have changed no setting. There has been no recent update. I even restored the system to a few days back before this problem began. The problem began uniformly across many websites at the same time: beginning with a reboot this afternoon - no ads this morning before restarting. Examples: Masterpiece Theater now studded with video commercials throughout a program; Huffington Post now plays as ad before a news video; MSNBC - where I could skip through the segments of any program now plays a commercial before each segment ... CNN more video ads, commericals are suddenly running on all websites where I NEVER encountered them before. NEVER. I even have those pulsating sidebar ads ... ads everywhere. I've tried changing privacy settings, going to sites to determine if my system can block websites and then I blocked almost 150 sites. No matter what I've tried it like suddenly being in TV commercial land. This is why I don't own a TV. What could cause a sudden, uniform, multi website ad flame up? As a user that clicks around many websites sampling snippets, having constant repetitive commercial videos clog up time and task is crazy making. I am desperate enough to ask McAfee for help. Save me. Please.

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.