User interface changes in some newer browsers have gotten some in the security community riled up. The issue is self-signed certificates. Some folks don't like users being told that their roll-your-own certificates aren't as good as the non-free ones. But the fact is that they aren't as good, especially when the overall population of users of web browsers is considered.
The https:// in a web address means that the web server has an SSL digital certificate, and this does 2 things for you. First, communications between you and the web server are encrypted (the certificate is used to construct the private key for the encryption). Second, the certificate contains an identity which is presented to the browser user; the user can look at this identity and decide if it is the right one for the web site and if it is trustworthy.
The tools to make these certificates are free and anyone can make a certificate that says they are "Citicorp, Inc. New York, NY", so how are you supposed to know the difference? The answer is in trusted certificate authorities. Web browsers (and other software, such as Windows itself) come with embedded lists of these trusted CAs and their public keys. The idea is that these CAs, before they issue a certificate to an entity, check to see if that entity is in fact the company or individual it purports to be. Thus the CA is vouching for the identity of the holder of the certificate
When a certificate comes along the browser sees if it was signed by, and therefore issued by, one of the trusted CAs. If it is, then things are cool. If not, then nobody is vouching for the identity. Because the Internet is full of lying, thieving no-goodniks browser authors have decided that such certificates deserve a special level of scrutiny.
Internet Explorer 7 and Opera 9.51 both react to such a page with a warning that the page's certificate is not quite right and that such a certificate is sometimes used by malicious pages in order to trick the user. They ask if you want to continue and, if you do, you are allowed to go to the page. Internet Explorer makes the address bar red for the page and puts "Certificate Error" in the right-hand part of it.
Firefox, on the other hand, shows a much more urgent warning with much more technical stuff thrown at the user. If the user wishes to go on to the web page they can't just go there, they first have to create an exception rule, a multi-step process which includes many more warnings.
For a demonstration of how this works in the 3 web browsers see our Slide Show: Browsers And Unsigned Certificates.