Spiderinabox provides an out-of-the-box solution for secure browsing on Mac systems. It includes a basic preconfigured installation of OpenBSD prepared as a VirtualBox image loaded with Firefox and a few scripts and programs to launch it and invoke Firefox over a secure connection. The visualization of Firefox is done on the OS X side using XQuartz, so you don't need to exit your working environment and switch between OS X and OpenBSD. Firefox runs encapsulated in OpenBSD and doesn't have access to OS X in any way.
CreditCardNanny is a Web browser extension for Google Chrome and Mozilla Firefox that detects forms that otherwise look secure (with a proper and valid SSL certificate, encrypted, and seemingly safe to enter sensitive data such as credit card details) but actually then submit this form data in a cleartext email to the site administrators. This extension detects such forms and warns you of the possibility that sensitive information may be exposed in clear-text, putting you and your sensitive data at risk. You can test the extension by browsing to https://cc-nanny.appspot.com/test-secure-page, a dummy credit card form that uses a form emailer script, a very common pattern used on an alarmingly large number of Web sites.