No certificate problems here with www.protonmail.com.

What kind of error page do you see?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

You can check if there is more detail available about the issuer of the certificate.

• click the "Advanced" button show more detail
• click the blue error text (SEC_ERROR_UNKNOWN_ISSUER) to show the certificate chain
• click "Copy text to clipboard" and paste the base64 certificate chain text in a reply

If clicking the blue error text doesn't provide the certificate chain then try these steps to inspect the certificate.

• open the Servers tab in the Certificate Manager
  • Options/Preferences -> Privacy & Security
    Certificates: View Certificates -> Servers: "Add Exception"
• paste the URL of the website (https://xxx.xxx) in it's Location field

Let Firefox retrieve the certificate -> "Get Certificate"

• click the "View" button and inspect the certificate

You can see detail like the issuer of the certificate and intermediate certificates in the Details tab.

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Here is the error from the page which reported certificate issue

https://mail.protonmail.com/

Peer’s Certificate issuer is not recognized.

HTTP Strict Transport Security: true HTTP Public Key Pinning: false

Certificate chain:

MIIFvjCCBKagAwIBAgIJAIcrLKktnw5GMA0GCSqGSIb3DQEBCwUAMGAxLTArBgNV BAMMJEJpdGRlZmVuZGVyIFBlcnNvbmFsIENBLmF2ZnJlZTAwMDAwMDEMMAoGA1UE CwwDSURTMRQwEgYDVQQKDAtCaXRkZWZlbmRlcjELMAkGA1UEBhMCVVMwHhcNMTcx MTMwMTIwNzM2WhcNMTkxMTMwMTIxNzAwWjCCAQsxEzARBgsrBgEEAYI3PAIBAxMC Q0gxGDAWBgsrBgEEAYI3PAIBAgwHR2Vuw6h2ZTEYMBYGCysGAQQBgjc8AgEBDAdH ZW7DqHZlMRgwFgYDVQQPDA9CdXNpbmVzcyBFbnRpdHkxHjAcBgNVBAUTFU5UUkNI LUNIRS0zNTQuNjg2LjQ5MjELMAkGA1UEBhMCQ0gxEDAOBgNVBAgMB0dlbsOodmUx GDAWBgNVBAcMD1BsYW4tbGVzLU91YXRlczEfMB0GA1UECgwWUHJvdG9uIFRlY2hu b2xvZ2llcyBBRzETMBEGA1UECwwKUHJvdG9uTGFiczEXMBUGA1UEAwwOcHJvdG9u bWFpbC5jb20wggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQC54kzgctr4 xi0yGHatU1tpjGe42agvVnEXOKaSmsAAyqrMdTQl5LFjJttMgvSWSeVCId1udkI3 SGAaw+DaN6rauXsL4FYySlR1fYFETm0sRsx8MPR1vbU3Scku5JmD+x/QUYfRCfFY Own/KMO1U7PAX6xIlwwBG/ZGJWW0rjVB11ixqZDEa3TbuNvW7viQ8D1vXnDbWL0X ByLWZpI8T6jAe9pdZz3XGAblkqsSaVC3V2Ey8cCxOjtlZyMcNcps5xOrxHvC7Sru Fzd7omYdJpRwtwSdUc6KMTScJ3gQeS1WS9C6bpcLdBHKniB/kRSWjdmYFXruv6ou zmeSmc0/p4CXAgMBAAGjggHMMIIByDA2BgNVHR8ELzAtMCugKaAnhiVodHRwOi8v MTI3LjEyNy4xMjcuMTI3OjM5MzkvOGNmNmEuY3J0MIIBjAYDVR0RBIIBgzCCAX+C DnByb3Rvbm1haWwuY29tghJ3d3cucHJvdG9ubWFpbC5jb22CE21haWwucHJvdG9u bWFpbC5jb22CE2JldGEucHJvdG9ubWFpbC5jb22CFXNlY3VyZS5wcm90b25tYWls LmNvbYIXY2FsZW5kYXIucHJvdG9ubWFpbC5jb22CEm9sZC5wcm90b25tYWlsLmNv bYINcHJvdG9ubWFpbC5jaIIRd3d3LnByb3Rvbm1haWwuY2iCEWFwcC5wcm90b25t YWlsLmNoghFhcGkucHJvdG9ubWFpbC5jaIITc3RhdHMucHJvdG9ubWFpbC5jaIIQ YXBpLnByb3RvbnZwbi5jaIINcHJvdG9udnBuLmNvbYIRd3d3LnByb3RvbnZwbi5j b22CFWFjY291bnQucHJvdG9udnBuLmNvbYIFcG0ubWWCCXd3dy5wbS5tZYINYWNj b3VudC5wbS5tZYIOcHJvdG9ubGFicy5vcmeCEnd3dy5wcm90b25sYWJzLm9yZzAN BgkqhkiG9w0BAQsFAAOCAQEAAgN50dl1xVJYMOPaNSUJT+RSN1nX+JTQSnj8gtFD b8/26K771VhjAwznP/bVUT8hKBt/a05gCgTDs4Zx2QKwsLIjv66KddZ1J8EP8e1W UHc+m3m6+abthO0J6FoL4OzalAh6FCnh0314Omt2Jc6pmlg1rJ0wqS1n3Ii1ahgR XVllsjhGQNqbW/1D5CCMmYyO7p+Ot/T7GLefkFbfEBtONKVL4oGut5N3aZweNxYm Q1TN4D/DhBkjAgMw0XKVYKfW5Dw+edb97I9aBUYjObyZGML5tZ48Od7U3HWgdOCG ThndmwCfQdaz7RQLhLV0JSCO1KwPsnKGLYJruG95R2WGiQ==

MIIDZjCCAk6gAwIBAgIJAPzlx93UHqQmMA0GCSqGSIb3DQEBCwUAMGAxLTArBgNV BAMMJEJpdGRlZmVuZGVyIFBlcnNvbmFsIENBLmF2ZnJlZTAwMDAwMDEMMAoGA1UE CwwDSURTMRQwEgYDVQQKDAtCaXRkZWZlbmRlcjELMAkGA1UEBhMCVVMwHhcNMDkx MjMxMTQwMDAwWhcNMjgxMjMxMDI1MTAzWjBgMS0wKwYDVQQDDCRCaXRkZWZlbmRl ciBQZXJzb25hbCBDQS5hdmZyZWUwMDAwMDAxDDAKBgNVBAsMA0lEUzEUMBIGA1UE CgwLQml0ZGVmZW5kZXIxCzAJBgNVBAYTAlVTMIIBIjANBgkqhkiG9w0BAQEFAAOC AQ8AMIIBCgKCAQEAueJM4HLa+MYtMhh2rVNbaYxnuNmoL1ZxFzimkprAAMqqzHU0 JeSxYybbTIL0lknlQiHdbnZCN0hgGsPg2jeq2rl7C+BWMkpUdX2BRE5tLEbMfDD0 db21N0nJLuSZg/sf0FGH0QnxWDsJ/yjDtVOzwF+sSJcMARv2RiVltK41QddYsamQ xGt027jb1u74kPA9b15w21i9Fwci1maSPE+owHvaXWc91xgG5ZKrEmlQt1dhMvHA sTo7ZWcjHDXKbOcTq8R7wu0q7hc3e6JmHSaUcLcEnVHOijE0nCd4EHktVkvQum6X C3QRyp4gf5EUlo3ZmBV67r+qLs5nkpnNP6eAlwIDAQABoyMwITAPBgNVHRMBAf8E BTADAQH/MA4GA1UdDwEB/wQEAwIBBjANBgkqhkiG9w0BAQsFAAOCAQEAMEpWNiEX u9++P0jf+DT/3Vq3krpmtciWqv+2PjeZYvsB/xrTOH+PJnIaAnuEViPkD13mSVdQ LfLaenyumDLzUzz9PkUfqHFX9D4fM24nGx5NceYKLCX8y4X4K8mnX+ldjT2oy5J+ Qpv2DA/7VPDAKh+nDAsiHSHzELqw6fvktDYWml5W+vxCmTApYuvdfKqTPnF75L82 UJCdxcNmSPoLzGfCdMTiMBg30sEadOetApwteb5m/G4ot7B9Ja1Tctx551DMTXaP mlPyu1sGFWe3Y4WamXJIJMwhE4T74wiIQO+Np5JvY5MkF/U107o1fwM3DBiaQyQA 196OSN8+5eCH5w==

First certificate is ProtonLabs.

Second one is the likely culprit as it is from Bitdefender.

https://support.mozilla.org/en-US/kb/error-codes-secure-websites

Yes when i disable BitDefender it allows me to access ProtonMail

But my BitDefender is AntiVirus Free Edition there is no option as you described in Dashboard etcc...

How do i keep BitDefender and allow access to ProtonMail with FireFox?

And i made sure to add exclusion in Certificate in FIreFox See attached

no difference i restart the FireFox and while BitDefender is Protection ON i cant access ProtonMail

What else can you suggest?