I'm looking at this from the perspective of the plugin developer not the user of the browser, so I'm interested in what developers think is the ideal interface for plugins to a browser. For example: Plugins can reorder, create and destroy Tabs, Plugins can draw behind and in front of Browser pages etc.
I'm particularly concerned about the security aspects
A web browser should have a firebug plug inn which help the developers.Concerned about security browser should block Phishing.
The major thing that I look for right away is how easy it is to open a preference pane, new windows, and the security preference variables.
I also like the ability to iterate through tabs, searching through various aspects such as page currently displayed within each of those tabs.