The reason for this is that because Mozilla employs a rapid release just like Google Chrome, but since building Mozilla packages requires more attention (due to increment of dependencies), it's probably the reason why it's now added into testing/. It's also a proper place for public to test the new version if they are an extension developers or beta testers.
So, here's the latest addition on -Current:
Sun Feb 12 23:26:00 UTC 2012 testing/packages/mozilla-thunderbird-11.0b1-i486-1.txz: Added. testing/packages/seamonkey-2.8b2-i486-1.txz: Added. testing/packages/seamonkey-solibs-2.8b2-i486-1.txz: Added.