Does anybody have an idea how to control the firing angle of a triac using a uP?

 

In theory, yes. The uP will be a somewhat bust animal tracking the variables to do good control. Since the AC line frequency is invariant, the uP can track firing angle with a zero crossing detector input to begin a timing cycle.

After that, it is a matter of the desired application. For lighting control, firing the triac at the same phase angle during every cycle is adequate. If the triac conduction is intended to maintain a constant voltage through a rectifier to a load, then an additional input will be necessary to monitor the voltage and adjust the conduction period.

For some additional material about using triacs, here is a link to out Ebook section that talks about triacs - http://www.allaboutcircuits.com/vol_3/chpt_7/6.html.