What it means
HTTPS-Only Mode warns when Firefox cannot upgrade a site from HTTP to an encrypted HTTPS connection. A certificate warning arises when a site attempts HTTPS but the certificate or connection cannot be validated. These are different failures. Adding an HTTP exception accepts an unencrypted connection to that exact site; it does not repair or authenticate a bad certificate on an HTTPS page.
A real-world example
A local router only serves
http://192.168.1.1 and triggers HTTPS-Only. A bank page presents https:// but a certificate warning. The first may be a constrained local-device exception; the second is a reason to stop and investigate rather than forcing a login.
What to do
Verify the destination independently. Use temporary HTTPS-Only exceptions only for known local services when necessary. Never enter financial or recovery credentials through a public HTTP page to clear a browser warning.
The distinction that matters
Encryption means traffic is protected to a server, not that the organization behind it is honest. A certificate warning can result from a clock error, interception or misconfiguration; bypassing it is a separate and often more serious decision. A temporary HTTP exception should be removed after the local task. If a familiar public site suddenly stops supporting HTTPS, treat that as a reason to verify its address and service status before supplying any data.
Mozilla HTTPS-Only behavior
