At our SAML based federation, we use this plugin extensively to study and debug SAML authentication flows between various IdP's, SP's and our gateway. We also recommend it to our customers, as well as service providers and developers as an insightful way to understand the SAML flow and identify the source of compatibility problems between products. The im/export functionality allows us to help debugging problems that we ourselves are not experiencing. Extremely useful and a huge timesaver.

For those that work with SAML this plugin is very helpful, however I can think of a few feature that would make it much better, for example:
- toolbar icon
- xml syntax highlighting
- show only SAML messages
- export message to xml file
- ability to specify credentials to validate & decrypt messages
- mozilla addons validation (loose the experimental status)

Nevertheless, I still use it alot! =)

Tools seems good, although it does not show the decoded SAML AuthnRequest?

Very handy add-on that works a bit like LiveHTTPHeaders only it's SAML(2?) aware.
Makes debugging SAML flows much easier.