You can check for issues caused by a corrupted or incomplete Visual C++ installation that is missing runtime components (Redistributable Packages) required by Firefox, especially VCRUNTIME140_1.dll.

See "Visual Studio 2015, 2017, 2019, and 2022":

• https://docs.microsoft.com/en-US/cpp/windows/latest-supported-vc-redist

Download the latest vc_redist.x64 and vc_redist.x86 files.

• https://aka.ms/vs/17/release/vc_redist.x64.exe
• https://aka.ms/vs/17/release/vc_redist.x86.exe

You need to reboot the computer.

• https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems

Boot the computer into Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

Thank you for your answer. I installed the packages. Firefox still didn't start. However, in Safe Mode Firefox seems to operate normally. Any ideas on why that is the case / how to fix it? I never had an issue like this before.

It sounds like your Microsoft account has parental controls enabled which limits what browsers you can use, you could try to get this limitation removed or create a new account. Renaming firefox.exe to something else can get it working but this will break DRM protected videos.