My mom can't figure it out, either
Well, I am with your mom!!
I can confirm this happening only sometimes, and it was a PITB till I worked it out. I am not sure it is related to Thunderbird alone though. I never use Thunderbird, even remove it if it is there. However, I have had it happen when Opera is installed with Firefox or if Iceweasel & Firefox are both installed.
You have to do the check with the box ticked, then untick the box or the default will not hold. This works so I never pursued 'why' with any gusto, but it must be some combination of events being present.
The external link thing might be the key?