I recently noticed some cold air around the bottom of our fireplace
hearth, which is brick and mortar. A quick look showed that the
mortar under the bottom brick has worn away, in many places quite
deeply. Should I fill with mortar, or could I use some other sort of
sealant (foam?) to fill this space? If the former, any tips for
someone who has never dealt with bricks and mortar?

Thanks!

-Ben