Yes to start running KDE, you would select KDE as an app in the menu of OS2008. It doesn't go straight into KDE.

Konqueror is the webbrowser in KDE, IMHO it is just as good as Firefox would be in this environment. Konqueror also doubles as a file manager/browser so that minimizes the need to install extra apps. Konqueror is definately better than Microb, only down side is that you loose flash plugin in Konqueror.

To more directly answer your question - I don't know if you can add FF to the KDE install or even if it would work - Penquinbait will have to answer that one.

I know KDE is slower than OS2008 but the tremendous improvement in functionality, list of apps, customization, etc. more than make up for it. Sorry, I can't give you an exact percentage for the change in speed because I haven't used Maemo since December (that's when I first installed KDE). Also the speed difference depends on what application you're loading (this is where the slow down is evident) once an application has loaded there isn't really any noticable difference in speed. You can still run the majority of Maemo apps in KDE.