A bug was discovered back in mid 2012 that caused the Firefox browser window to lose focus under certain circumstances when Flash contents were played in it.
What this meant is that Firefox users had to click on the browser window to make it the active window on the system again even if it was the active window before.
The system the bug was encountered on was a x86 Windows 7 operating system with Flash 11.3 initially but other systems such as Windows 8.1 are also affected by it according to user reports.
