Comment # 1 on bug 1022200 from Wolfgang Bauer

Can't reproduce this here.

Maybe your ~/.kde4/share/config/kdeglobals got corrupted somehow?
Try to delete it (or try a fresh user account) and see if the issue persists.

Also, systemsettings5 syncs back its settings to KDE4. So try to change the
browser there, otherwise it may revert your KDE4 browser setting if you change
some other settings.

Another thought: are you actually using Firefox?
It has a check whether it is the default browser and offers to set itself as
default browser if not. This currently affects the KDE4 settings specifically,
as the KDE integration is still KDE4 based. And I think it may do this
automatically even, depending on its settings.