hi, firefox doesn't have any self-destruction mechanism that would kick in this way every day. it is likely that you have to look into external factors on your system that are causing this. are you using any special security/sandboxing-software or cleanup-utilities?

Hi, thanks for the reply. The only security software that I'm using is F=Secure virus checker, Malwarebytes Anti Malware and CC Cleaner. This is a brand new Windows 10 PC that I've just bought last week and it's been doing since day one.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.