This article describes why the The certificate is not trusted because the issuer certificate is unknown error message may appear when trying to visit secure (https) websites in Firefox.
For example https://code.google.com/p/end-to-end/
In the URL above, the domain is “google.com” and “end-to-end” is a sub-site. The problem shows as the following:
In the screenshot above, the site name that could not be verified was code.google.com as the syntax of the error is:
(site name) uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (Error code: sec_error_unknown_issuer)
Solution of “certificate is unknown”:
The above error is normally seen when the file cert8.db in your profile folder has become corrupted.
In order to fix the problem, we will have to delete the file while Firefox is closed.
To do so, we will need to find and open the profile folder. This can be done by clicking the menu button , clicking help and finally selecting “Troubleshooting Information” as seen below.
Once the troubleshooting information page opens, select the “Show Folder”
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That should fix the error message “400 Bad Request”, if you have any questions or concerns please leave a comment below. You can also follow me on Twitter, add me to your circle on Google+, or like my Facebook page to keep yourself updated on all the latest of Hardware and Software reviews.