In recent weeks, on both machines, Firefox occasionally disappears, about half a dozen times in total, and when I restart Firefox the window I had been using doesn't come back. The History tab has no trace of the lost window, nor of any of the tabs it had contained. It takes me a long time to try to "reconstruct" the lost window and tabs from memory, and I'm only ever partially successful. So these incidents are extremely annoying. I use Avast software to protect my computers, and I tried to blame Avast for the above happening, but they denied being responsible. I allowed them to remote access my desktop PC, and they said that they could detect nothing wrong. The problem has happened again this morning on my desktop PC, hence this report. The two computers are not linked in any way other than accessing the internet via the same cable out of the house, so I don't think that some kind of "cross-infection" could have caused Firefox on both to become unreliable. Of course, both machines automatically update their versions of Firefox and Windows 10.