Firefox and Thunderbird stopped to open the application window same day (about a week ago): only firefox.exe and thunderbird.exe processes start. Task Manager also only lists the processes, no applications. This process is very hard to kill, Task Manager can't end it, nor elevated rights Command Prompt TASKKILL with /FE, NirSoft NirCmd nor PSTools pskill64.exe nor Process Explorer. I have also tried ending all the other processes first I can think of, including explorer.exe, does not help. I'm testing with two administrator level accounts and I'm mostly working this problem with Firefox now, although Thunderbird shows the same symptoms. My system is a clone desktop computer Windows 7 PRO 64bit UK, AMD:n Phenom II CPU, GeForce 1050Ti display adapter. As antivirus I have Avira Free Antivirus, and I have some suspicions it may be the root of the problems this time. For couple of weeks ago it updated with a new user interface and installed new components. To test that, I tried to uninstall Avira. Each component needs to be uninstalled separately using Windows Programs and Features: two components uninstalled cleanly, one component left an open minimized window that one couldn't pop open nor close. Antivirus and VPN do not uninstall: after you are asked do you want to uninstall and answer YES, nothing else visible happens. If you try to start another uninstall job, you get a window telling you can't start another as one program is already being uninstalled or changed. And waiting doesn't help. I have a support question about it in Avira forum. Also last week, Flash Player installation failed with 89% done, tried twice and with same result. And the newest GeForce Graphics adapter drivers installation failed also. Both of those usually always install succesfully. Microsoft System File Checker SFC /SCANNOW does not find problems, nor does Windows disc chechker tool. I also run: File and Folder Troubleshooter winfilefolder.DiagCab and Microsoft Program Install and Uninstall troubleshooter MicrosoftProgram_Install_and_Uninstall.meta.diagcab ja Program_Install_and_Uninstall.diagcab. Avira support recommends the Install and Uninstall troubleshooter to get rid of some old Avira rubbish. Also compared Windows folders rights to default rights listed in some web site: did not find differences with the couple of dozen folders I compared. Data Execution Prevention (DEP) according to Process Explorer is enabled looking at Firefox.exe process properties. I know command lines to change DEP settings and found it in System Performance Properties settings also, but have not touched it. I've removed Firefox and Thunderbird parent.lock -files, with Firefoxilla both from profile folder C:\Users\USERNAME\Application Data\Mozilla\Firefox\Profiles\PROFILENAME and temporary folder C:\Users\USERNAME\AppData\Local\Temp\firefox before trying to start Firefox again. And since I can't kill the process otherwise, it also means after rebooting the computer. Found an advice opening Firefox with SHIFT-key down, should open a safe-mode of the browser. Well, opened only the process in my case. There are no previous session related saves (sessionCheckpoints.json, sessionstore.jsonlz4, sessionstore.js) in the profile folder any more, and the sessionstore-backups folder is empty. I removed the Firefox installation, rebooted, and installed the 73.0 version 13.02.2020. It behaves similarly: only the process starts. Added -p parameter to the Firefox shortcut, and in Windows SAFE MODE was able to create a new profile: in normal mode Windows the window for creating/changing profile does not open. Firefox doesn't open application window in Windows safe mode either, also tried to run it as administrator option, and pressing SHIFT key, but neither helped. The newly created profile folder only has one file: times.json. When installed Firefox again, it wasn't a clean installation: the Firefox folder was empty of files, but I did install into the same folder. Also did not edit register nor delete the profile folder. Which is what I'm gonna try next. However, Thunderbird does not use Firefox's profile, so why does it stop opening the application window same time with Firefox? I don't think Firefox's profile folder can cause all these installation problems and Thunderbird problem. My bet is more likely Avira.