I have not encountered this before. I cannot see my local sitecore website unless I keep Fiddler running... Is there a way to decouple and uninstall Fiddler now?
I tried removing the DONOTTRUSTFIDDLER root certificate, tried using the tool in Fiddler to 'remove interception certs', but it just keeps giving that SEC_ERROR_INADEQUATE_KEY_USAGE error unless I reinstall Fiddler and trust the certs.. otherwise I cannot get work done and Fiddler just uses resources in the background for no reason.
How can I get rid of it like it never happened?
Sitecore 9.3 had a bug where the Identity Server certificate would throw SEC_ERROR_INADEQUATE_KEY_USAGE error in Firefox. The error was caused by wrong certificate usage flag set by the Sitecore Installation Framework (should be Digital Signature instead of Data Encipherment). This was fixed in a later version of SIF.
As version and error logs are not shared the above is the most likely explanation. Alternatively you might be experiencing a very similar browser/certificate issue.