browser lifetime functionality is a MUST for me. Having the ability to extend somebody else's website (e.g., Google Page Preview plugin) or write javascript code that does interesting things with any webpage (e.g., bookmarking as it was pointed out above) is something that can open up lots of exciting use cases, as opposed to just have my own page spiced up with more functionality. Greasemonkey is fine to experiment... but not viable for real deployment.
I hope you guys will put thought and weight on this idea!!!
The first step to moving to browser lifetime is identifying what features should be supported. This is a bit trickier that what we've done thus far because so far we've been taking a viable subset of what's possible in NPAPI and ActiveX and making it work everywhere. This next step would be analyzing the add-on frameworks of all browsers in existence and identifying an interesting subset there.
Javascript Injection similar to greasemonkey could be an interesting place to start. Allow services to provide a regex to match on hostname and javascript to inject?
The dangers of extending beyond what's possible with plugins is that we might have to sacrifice the ease of initial installation, and this risk is why we've prioritized other more basic features (i18n, l10n, stability, portability, and distribution size) above browser lifetime.
That point aside, it seems like the first browser lifetime features would be js injection and the ability to intercept a web request at page load time... Anything else you think would be interesting to start with?
best,
lloyd