I did this twice.  I checked that I entered the info correctly.  I entered the code within the 5-minute timeframe.  Both times I got the same message - the code is invalid.  It will not verify my secondary email address.  
   Very frustrating.

I am trying to add a secondary email address (a Zoho email address) to my Firefox account (my primary is a gmail address). I receive the verification code in zoho email. I enter the verification code into the required space for Firefox to verify and accept that I am me and that's my email, and I get an error message that the code is invalid. I did this twice. I checked that I entered the info correctly. I entered the code within the 5-minute timeframe. Both times I got the same message - the code is invalid. It will not verify my secondary email address. Very frustrating.