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.