I have both a desktop and laptop, and both experience this problem. On laptop, if I have an open browser, and close the lid, and bring it back up, Firefox will be frozen and not respond. I can open a new browser window, and it works fine, but I have to right click on the app in the task bar and close it to get the original browser window to go away. My desktop doesn't go to sleep, but the screen is set to turn off after an hour. I lock my PC at the end of the day, and most mornings when I come in the browser window is unresponsive. I have to start a new browser session. I do not have this problem with Chrome. In doing some web searching, I tried disabling hardware acceleration, and clearing cash and history, and it seemed to get better for a few days, but now it's happening again.