I've found that closing Firefox and reopening didn't always work. And when it did, it was a little slower. Also, if your mouse is frozen, some people don't know how to get to the Start button on Windows to reboot.
When in doubt...pull it out (power cord). It's worked for me since Win95, but then I've always had a strong anti-virus/firewall from day one.
Also, on the rare occasion I don't have Internet access, I simply pull the power cable from the router (after checking the modem lights) and plug back in. Maybe I should buy a new router.
Back in the XP days, I learned that holding down the power button should eventually cause a destop and laptop to shut down.