That's strange. I don't think websites can call up the Scratchpad. Is any email relevant content displayed in the Scratchpad, or just the generic eight-line introduction to Scratchpad? Shift+F4 is the keyboard shortcut to call up the Scratchpad. Maybe there is a conflict somewhere.

Unfortunately, I did not find a quick and reliable way to turn off the Scratchpad feature in my search; solutions from 2011-2013 did not work in Firefox 39. If it is getting triggered by Shift+F4, you might need an add-on to break that connection. However, I'm not sure whether that is the right route. Can you figure out anything that might be triggering it?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes