A secure-connection warning means Firefox could not establish the expected protected connection. The useful next step is to collect enough detail to distinguish a website problem from a computer or network issue, while leaving the warning in place.

Check the address for a typing error and record the exact hostname and time. Open Advanced if needed to read the error code and description. This displays diagnostic information; you do not need to proceed through the warning to collect it.

Identify the scope

Check whether the computer’s date, time and time zone are correct. Then try a different familiar HTTPS website without entering account details. Note whether the failure affects one site or several. Mozilla describes possible causes including website configuration and software or networks that intercept secure connections.

If another approved browser is available, compare the same address there and record the result. A successful load elsewhere is a clue, not proof that bypassing Firefox’s warning is safe. Different applications can use different trust or connection settings.

Send a useful support description

For a work device, provide the error code, affected hostname, approximate start time and whether the problem changes between authorised networks. Avoid sending a full screenshot containing private tabs, account information or network credentials. Let the responsible administrator check managed certificates, proxy settings and security software.

For a single public website, contact its operator through a known channel and describe the warning. Do not install an unfamiliar root certificate or turn off certificate validation simply to make the page open. Resume sensitive sign-in only after the underlying problem has been resolved and the browser accepts the connection normally.

Source: Mozilla Support.