I've been moving between computers and used a time machine back up to bring over everything, including my Firefox profile. Everything was fine but about 2 weeks ago the computer seemingly crashed while I was absent and on the restart and ever since, just the one window, of 3, that I had open stays on screen. I have tried seeing all windows and looking on separate desktops but all to no avail. When I quit Firefox it clearly recognises all the windows as the tabs count is correct but only the 1 window was ever visible. I did all the madness of renaming the sessionstore file and deleting the recovery json files and replacing the previos json file but nothing has changed the outcome. I tried the Henderson?!? website to get a list of everything but that just looked like it was eternally thinking and never concluded. It's highly likely the workload was exceptional and might well have completed one day but it seemed to be stuck. If it helps, I'm on an 10 core 3.6ghz iMac with 64Gb of RAM and I'm using the latest Firefox version. Before anyone asks, I had an obscene amount of tabs open as I have many projects on the go and different windows are used for different things with multiple tabs in each window. The highest total tabs I've ever had was 541! I know, I know.