To set your default browser, go to ‘Settings’ > ‘System Settings’ in the KDE Menu.
The second row, ‘Personal’, has an icon for ‘Default Applications,’ select that one.
In the left hand list select ‘Web Browser.’
Select ‘in the following browser’ and do not use the ‘…’ selector
Instead type the exact path to your browser followed by a space.
/usr/bin/firefox
Otherwise, KDE will attempt to cache the web page in /var/tmp/kdecache-<yourusername> and then open it in your browser of choice. Even worse, KDE never clears this cache.
