Here are some thoughts on those test frameworks :
- PHPUnit : Good for back-end work, but weak with front-end (browser) work
- SimpleTest : Same as PHPUnit
- Watir : Very mature and popular browser test app, but requires knowledge of Ruby scripting language
- POW : Some potential to automate FireFox, but fairly small scale. Not a testing framework
- iMacros : Same as POW
- AIR : Good app development framework, but doesn't really have a automatable browser. Not a test framework.