The certificate for www.bitaddress.org is issued by PositiveSSL. Chrome trusts them (PositiveSSL), firefox does not.
There are an awful lot of issuers, and different browsers are bound to have slightly different lists.
If you trust them or not is up to you. If you trust Chrome's security, then you can trust that certificate.
If you "add exception", then BEFORE confirm security exception, you can "View..."; you can check the fingerprint (SHA1 & MD5), and if they match, then the security is the same between Chrome and Firefox (not necessarily good, just the same between the two browsers). If they are different, then someone is doing something SERIOUSLY FLAKEY!
Of course, you shouldn't trust a website that throws up a warning, just because some guy on the internet told you to :)