Browser Error or Hack?

If Firefox shows a warning of an untrusted connection at an https site, yet other people were able to go to there, and tested it with 3 different browsers, how likely is it that this is a browser error and not a hack? Someone I asked about it says it's very difficult for someone to impersonate an https site, though not impossible. I’m very careful where I go and who I trust, also I update my Mac and browser as soon as they come in.

UPDATE: I just got a reply from someone who checked it out, it's the site itself, not hacked, just a tech glitch! BIG SIGH OF RELIEF but it's VERY important that we check these things out so others will know it's not a hack, then they won't panic.

We weren't ging to panc as you did not advise the site.i