Change your default browser to another then back to Firefox, this should toggle that preference:

• Make Firefox your default browser

PPLT said

Clicking on email links opens Firefox but not the link. Firefox must be open first before and I must click on email link again to open it. How can this be resolved?

Thanks but unfortunately it did not solve the problem. I switched to SAFARI and then back to FIREFOX as suggested, but while email links work with SAFARi they still don't work with FIREFOX.