Due to memory problems I closed Firefox whilst the
Group My Tabs (Ctrl+Shift+E) feature was displaying the multiple tab groups. After 20 minutes the process still hadn't finished so
killall firefoxed it.
tabview.js that hangs unless I click
Stop at the warning screen. If I do hit
Stop then I can no longer open the tabview, so I can't get to any of the tabs except in the subgroup I was last looking at.