Text follows in case it disappears:
"I use firefox and this used to happen to me too. The solution is very simple.

This used to happen to me very often. If you have Windows, as soon as
the browser seems to disappear push Alt+Tab together at the same time
and continue pushing tab until you get to the Firefox icon. If the
Firefox Icon is not there push Ctrl+Alt+Del and bring up the Windows
Task Manager, then click processes and in the "Image Name" column check
for "firefox.exe" that will show us if the process is still running
(good and bad). If it is, watch it for a minute and see if the "Mem
Usage" stays the same. If it does then click on "End Process" with
Firefox.exe selected. This will close the process and allow you to open
firefox up again (sorry, your work is lost.) If the memory seems to be
fluctuating then that means that the window is still open, but somehow
hidden in your system. Try going to Start, then Run and typing in
firefox.exe. If it says the process is running so you can't open it, or
something of that nature then just follow the above instructions and
close the process. When you open firefox up again disable all of the
extensions. Then slowly add them all back, one by one. That is the
easiest way to find the extension that may be causing this. Good luck!"