I had a problem with Firefox 49.0.2 (64-bit) today, which is installed on my work laptop running Windows 7. I have the usual work related programs open, along with an instance of Firefox with 3 tabs open. The tabs were also work related and set to refresh every minute, which is how I normally run my laptop... nothing unusual.
Firefox appeared to be working, but the tabs stopped refreshing as if I lost my internet connection. I was unable to access any other site as well. I double checked and verified that my connection was stable, and then decided to close and reopen Firefox. But I could not reopen Firefox because Windows kept reporting it was still running. I tried closing it with Program Manager, but it was not showing Firefox on the list. After several failed attempts I gave up and rebooted my system. After logging in, Firefox worked properly again.
This is a clean system, completely free of malware. I have no idea what caused this, and I don't really know if it was an issue with Firefox or with Windows.