Firefox locks up when starting first time after restart in Windows 10

Hi coleburg, I searched the current Mozilla documentation, and this is a known type of Firefox startup hang. Mozilla specifically notes that interactions with antivirus or other internet-security software can cause Firefox to hang as soon as it opens. Because you are seeing it on both Windows 10 and Windows 11 machines, and Firefox works normally after that first launch, I would test these things in this order: First, start Firefox in Troubleshoot Mode. You can do this by holding Shift while starting Firefox, then selecting Open. Mozilla says this temporarily disables extensions, themes, hardware acceleration and some customisations. If the first launch works normally in Troubleshoot Mode, one of those is likely involved. If it still hangs, try clearing Firefox's startup cache. Go to Help > More Troubleshooting Information and click Clear startup cache, then restart Firefox. Mozilla specifically recommends this for some startup/UI problems. I would also check what antivirus or internet-security software is installed on the affected computers, since that is one of Mozilla's documented causes of startup hangs. Since this is happening on multiple machines, could you tell us which antivirus/security software they have in common and which Firefox version they are running? That would be a useful next step before reinstalling Firefox.