Pros
1) We will have the most popular open source browser, only chrome (not chromium) has more users than Firefox and it's not fully open source (it had flash built in)
2) Very active upstream.
3) Familiar experience for first time users, Firefox tries to look similar on all platforms.
4) Better support for internet video
5) Much better support for the latest standard (HTML 5), if you compare Konq to Firefox in http://html5test.com you will be shocked.
6) Support for addons to extend the functionality.
7) Higher number of users and developers mean bugs and security vulnerabilities gets found and fixed faster.
8) Firefox is faster than Konq.

Pros
Less integration with Plasma desktop and KDE apps.

Mustafa Muhammad