thunderbird does not start if user is not member of "wheel" group

[Trust_No_1]

Hello everybody.
sorry for the stupid question, but I'm facing a strange (for me) problem.
Let me explain:
I have a nice FreeBSD workstation (running FreeBSD 10.2-RELEASE-p10)
that works very well for me.

On this ws I have two users (say user1 and user2), but only one of them
(user1) is in the wheel group.
The second user is just in it's own group because I do not use it to
administer the ws.
So I have:

root at dukefleed:~ # groups user1
user1 wheel

root at dukefleed:~ # groups user2
users

If I launch thunderbird with user1 all is OK but, if I try to start
thunderbird as user2, I receive the following error:

user2 at dukefleed:~ % thunderbird
Could not find the Mozilla runtime.

No matter if I try to launch it via the full path, cd into
/usr/local/bin,... it does not start.

The problem disappear if I add user2 to the wheel group.
Providing that I could add user2 to the wheel group and solve the
problem, I would like to know why I have this behavior.

By the way: on the same ws, firefox DOES start without problems for both
the two users, and in particular (for user2) it starts even if I do NOT
add it to the wheel group.

Sorry but I googled a lot without finding any link to a solution...

Do you have any ideas?

I use thunderbird 38.5.0

Thank you very much and kind regards.

Mauro.