Hello.
Northern lights can be seen well between appr mid Sept to end of March. Can also be good early Sept and early April.
All months can be very bad, and all months can be very good. Solar activity and coastal weather are both very unpredictable, so you can not plan ahead which dates that will be good. The most important for you if you want to see good northern lights, is to have enough days in Tromso, waiting for a good day with lights activity and clear skies.
All months have advantages, and all months have disadvantages. We have northern lights in average every second day, so our answer is; have enough days. It does not matter which month you come, as long as you have enough days here.
Regards,
Øystein