When you clear the clipboard, the windows clipboard handling code will corrupt random process memory of applications that check if anything is on the clipboard.
Firefox was clearing clipboard on exit from private mode. My application was calling Clipboard.Clear() (csharp app). The display of the context menu is the exact moment when the corruption happens.
THe workaround is to not clear the clipboard. I replaced it with Clipboard.SetText(" "); No crashes since then.
Another workaround is to paste anything to clipboard, type some text from notepad, select and paste. This, at least prevents future corruptions, if you happen to have some program which clears the clipboard.