Usually power companies deliever same mains frequency throughout country. For example here in India 50Hz is standard.

But you can make your devices independent of mains frequency. For firing control you will surely need mains zero cross detection [ interrupt ] & a system timer to give required fire delay.

I use PCA interrupt for Mains zero cross detection & Timer 2 in 8 bit autoreload mode as system timer. My system timer ticks 200 times every half cycle of mains. When system timer reaches desired delay I fire the thyristors. Every time I get PCA interrupt I reset system timer to zero. The advantage of using PCA in capture interrupt mode is I can exactly measure the Half cycle period using PCA capture registers and accordingly adjust reload value for timer 2 auto reload. Due to this what ever be the mains frequency the timer will always give 200 ticks in one half cycle. Using this technique makes control immune to mains freqency making it usable at both 50Hz as well as 60Hz also mains frequency variations which can be as high as 5% can be compensated.

Warm Regards,
Prahlad Purohit