I've viewed the posts, and there's no explanation as to what caused the problems, just a trick-or-treat bag of things to try. What is it about FF 62.0.3 that's caused it to happen ? I run 4 add-ons: AdBlock, Image Background Color Picker, Theme Font & Size Changer, Webroot filtering extension, and have some icons moved to the toolbar. I verified that the about:config privacy.resist thing is the default 'false', but there are 3 others that begin with privacy.resistFingerprinting which weren't mentioned and which I haven't adjusted. privacy.resistFingerprinting;false privacy.resistFingerprinting.autoDeclineNoUserInputCanvasPrompts;true privacy.resistFingerprinting.reduceTimerPrecision.jitter; true privacy.resistFingerprinting.reduceTimerPrecision.microseconds;1000 I have no idea if those have any bearing on the problem. I have tracking protection set to always, with no issues. Its not set for use with a tablet, and never was. Firefox is my default browser, yadda yadda. The only thing I do regularly is clear my cache, cookies etc. Previous versions of FF, on this machine and on my old one, never messed with my window size/position. As to why I hadn't had the problem until today, I'd been shutting down the PC with the browser open, for the convenience of returning to the tabs as I left them, like resuming work laid on an actual desk. Nice. Last night was the first time I closed everything before shut down in a while. My set-up is simple, so app conflict is an unlikely cause. I have FF, wordpad and notepad open at once. Haven't added any software since buying the PC. Uninstalled Office 365 a month ago or more.