Am Samstag, 15. März 2014, 12:34:14 schrieb David Bell:
> I would like to use Firefox as my default browser but there doesn't seem to
> be  a way of implementing this in <Control Centre>.

try
control-center->kde settings->file assiciations->text->html
(translated from german, might differ somewhat for you :)
there you can select multiple browsers, as well as the one with highest 
priority
HTH
Werner