Hi

I have had a look at that site and it appears that American Express have fixed it in the last day or so. I recommend trying it again as the site did appear and search for me without a problem.

If it is still not working for you, please come back here and we will see what we can do to help.

The HTTPS address doesn't work for me, either, but the link in your post using an HTTP address redirects to a completely different page:

https://www.americanexpress.com/us/content/search/

Maybe you can use that one for now until American Express straightens out the old link?

When I load the problem link full page (https://search.americanexpress.com/) I do get an option to add an exception. If you don't see that, it could be because the page is loading into a frame. Because you cannot see the true address of a framed page, the Add Exception button is not available in that case. To test this theory, right-click the page and look for a "This Frame" item on the menu. If it's there, choose Open Frame in New Tab and check whether you can add an exception there.

Now... I realize you trust American Express, but that isn't the question. Firefox is alerting you that the site you see in front of you isn't set up correctly. Wouldn't you normally expect a company like American Express to know how to set up a web server? So usually that would make you suspicious that perhaps you're seeing a fake version of the site. Millions of people are fooled every year, typing out their logins or opening up their "secure" sessions to criminals. So please add exceptions with caution.