The first thing I have to do when I log into KDE is move Firefox windows to the activities they ought to be in. One to Programming, another to Writing, Administration, Internet, etcetera. KDE deserves a browser that takes advantage of activities and other features of the Plasma Desktop.
The browser would have a prominent button that creates an association between a domain and an activity. For example when viewing http://bugs.kde.org I can choose to always view the site on my programming activity. Subsequently if I open a link to that site in KMail (on Communication) the desktop would automatically switch to the Programming desktop and open the link in a window there.
Additionally there should be a dialogue providing fine grained control over this feature and a means to easily put web bookmarks on activity desktops. Perhaps tabs/URLs could be dragged onto the desktop or a quick-launch style plasmoid?
Other KDE Integrations
Is it possible to add website accounts into Akonadi? Many forums run PHP Bulletin Board so they could all be addressed with one module. Other major websites such as Reddit and StackOverflow would also be worth creating modules for.
Additionally KDE provides standard locations for different kinds of file such as image and video which the likes of FireFox and Google Chrome completely ignore. Saving to these different paths would be a trivial change that makes saving documents that little bit more pleasant.
Finally, I don’t know how many people want this but a VIM emulation mode akin to Vimperator for FireFox is what my personal decision to use any browser comes down to.