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Email Problem

Hey, folks, I've got a little problem with the email on my system.  I'm
running debian/Sid, but the issue has been going on for quite some time
even when I was running straight Jessie.  I'm sure it's a configuration
thing, I'm just not sure where to fix it. Here's the deal:
When I'm in Firefox browser and press the share by email button on
various web sites, the email client that pops up is always mutt (which
perhaps I should just un-install as I never use it).  Mutt tries to go
through Exim to send the email, and I don't have Exim set up for
internet access (another issue that is on the to-do list for another
day).  What I would much rather have happen is that the email client
that pops up be Thunderbird, which I use exclusively for all my email
stuff.  Where is the setting that will change this behavior? I've looked
through all the alternatives, and they seem to be set correctly, but it
is entirely possible that I missed on.

As always, thanks for any help in advance.
WB5VQX -- The Very Quick X-ray