Does this occur with one particular video, or with any video, or gradually over time as you view more and more videos? In case one of your extensions is involved, could you test the same usage pattern in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: [[Troubleshoot Firefox issues using Safe Mode]]. You can restart Firefox in Safe Mode using either: * "3-bar" menu button > "?" button > Restart with Add-ons Disabled * Help menu > Restart with Add-ons Disabled Not all add-ons are disabled: Flash and other plugins still run After Firefox shuts down, a small dialog should appear. Click "Start in Safe Mode" (''not'' Reset). Any difference?