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How to add a exception for SSL certificate error

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I am developing my web application locally under the url When I am trying to access the url I get a "Secure connection failed - SEC_ERROR_INADEQUATE_KEY_USAGE" error which is normal as the SSL certificate is not a valid one. But I would expect to have a "Add exception" button as I can see on Chrome and Edge for the same url. I tried settings the config parameter "network.stricttransportsecurity.preloadlist" to false and delete the SiteSecurityServiceState.txt as found in But it does not work. I still cannot access that url and no "Add Exception" button is shown. I have FF 91 installed on Win 10.

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I called for more help.

From my research, it looks like a certificate issue.


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I know it is a certificate issue. The certificate on the development is self signed, thus not valid. I already said this in my initial question. But in this case I would expect to have a "Add exception" button to allow access of that url even if the certificate is not valid. Chrome and Edge do this for my url. Only Firefox does not show such a button. I tried later to host my website app under url and in this case FF showed the certificate error page but offered me a button to add an exception for this url. So the only difference I would see is that the url with a specific port makes FF to behave differently.


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