my humor is dark, and sarcasm is my middle name… and interweb is my other mother. i’m a sysadmin by trade so human-human interaction with words is not my strong side. my strong side is being a dick.
I like the idea of using the “title bar tabs” instead of “inner tabs”. However, sophisticated window stacking/grouping features would be needed therefore. Which would be a nice thing not only for WebPositive, so I hope it will be integrated into Haiku some day!
Two other suggestions:
Outside BeOS/Haiku, I’m using Opera. One thing I really like about it is its easy inpage search feature: you hit the dot key, a little popup appears at the lower left corner of the window, you type something and it’s highlighted immediately. (At least this is how it was done in Opera 9, in Opera 10 it is not so neat any more.)
Another thing I like in Opera is that the download list is shown in a tab, not in an own window.
Thanks for your wonderful work so far, I’m looking forward to further improvements! 
I’ve resently red about chrome extensions implemented in rekonq (KDE’s WebKit browser) http://kamikazow.wordpress.com/2010/05/23/kdes-webkit-browser-rekonq-gets-extension-support/.
The reason why they chose Chrome’s extension API is that it based on HTML, JavaScript and other web standards which makes it easier to adopt than Mozilla’s current XUL-based model.
I think this would be the most simple and efficient way to implement extensions in Webpositive.