Today I was trying to access a '''local HTTPS website''' of an embedded device (a Siemens S7-1500 PLC for the record) that is using a '''self-signed certificate'''. I immediately get the '''SEC_ERROR_REUSED_ISSUER_AND_SERIAL''' error and I wasn't able to go on (attached image #1, in Italian language). I've certainly browsed a similar PLC webserver in the past, so the fact that the device exposes a similar certificate serial number is not so strange for me, considering also it's self-signed. The problem is that I cannot follow the instructions of the Mozilla knowledgebase [https://mzl.la/3Ad3wj1 here] because '''I'm not able to identify the correct certificate''' of the website! In the attached image #2 you can see that the connection is detected as not encrypted, I suppose because only the form part of the HTML that is posting the data using HTTPS while the rest is using HTTP, but I'm not sure. If I click on the padlock icon (attached image #3) the info about the current certificate is not displayed. Then I went into ''Tools > Options'' and searched for "certificates"; as you can see in the attached images #4 and #5, neither in the ''Servers'' tab nor in the ''Authorities'' tab I can find any item that is related to ''192.168.10.2'' (the webserver IP address) or ''Siemens'' (the embedded device manufacturer). But unfortunately the error persist! Initially there was a ''192.168.10.2:443'' line in the ''Servers'' tab and I removed it and restarted Firefox, but it didn't solve, so at the moment the situation is the one depicted by the attachments. I had to use Microsoft Edge to access the website, but I would like to solve this issue and use Firefox instead. Is there anything else I can do to bypass the error? Why doesn't the error page provide a way to correctly identify the wrong certificate using its fingerprint and/or name? Or, maybe, is there a way to disable this check for local IP addresses or for items in a whitelist? Thanks for your help.