I'm trying to access my (European Parliament) Webmail from a Linux/Firefox machine at the following address and I get security warning messages explaining that the identity of the site cannot be verified (the error message is in french).

But this only happens with Linux/Firefox from one machine.

Here's the address:

https://webmail.europarl.europa.eu/

(and I'm trying to access it from my home, not from the EP).

And here's the detailed error message:

webmail.europarl.europa.eu utilise un certificat de sécurité invalide.

Le certificat n'est pas sûr car l'autorité délivrant le certificat est inconnue.

(Code d'erreur : sec_error_unknown_issuer)

So basically, if I translate, it is telling that the webmail.europarl.europa.eu certificate is invalid because the authority that delivered the certificate is unknown. I do only get this invalid certificate thing on Linux/Firefox.

From a MacBookPro running Safari, I go to what looks like the correct webmail login page.

From the same Linux machine, but using another user account and Chrome instead of Firefox, I go to what looks like the correct webmail login page.

So there are several possibilities, here are a few ones:

1. Firefox is right and my Linux box has been hacked
2. Firefox is right and detecting something that neither Chrome nor Safari is detecting (like, say, my router that may be hacked)
3. Safari on the MacBook Pro and Chrome on Linux are both correct and it is just Firefox on Linux that is wrongly stressing me when everything is normal.

How do I know which one of these possibilities (or any other) is correct?

How can I troubleshoot what is going on with either Linux/Firefox or with the parliament's webmail?