A few people have written in regarding the Firefox plugin SSL Blocklist

The tool has been around for a while, but they have added the ability to detect MD5 signed certificates and block access.  It might be a nice addition to the arsenal.  Whilst the address bars in FF and IE do seem to turn green when the site has a SHA signed cert (at least it did for the sites I tested), this might be a bit more obvious. You only get the padlock when the site is MD5 signed.

Mark H