Thunderbird default browser option

[Robert F. Stockdale IV]

I currently use Mozilla Thunderbird for email and Mozilla Firefox for 
Web browsing. When ever I click on a link in Thunderbird it open 
Epiphany web browser. Does anyone know how to change the default. 
Usually,  I have Mozilla Firefox open at all times and would just like 
it to open web page in new tab or window of Firefox.  Thanks in advance.
Bob