Thunderbird's copy of Firefox

Hi, When an incoming email has a link to a web page, it doesn't open up in my separately installed copy of Firefox, which has my password keeper and other useful tools. It opens up, instead, to a separate copy that is controlled, it seems, by Thunderbird. How do I get rid of that intermittently appearing Firefox. Instead of being able to click on the link and go to my installed Firefox, it's forcing me to copy the link and manually insert it into the installed Firefox. The third image is my active Firefox window. The other two (duplicates) is the Firefox screen I get when I click on a link. My Firefox install appears on the lower menu bar. The Thunderbird generated Firefox is only accessible by clicking a link in an email. I hope there's a solution. Thanks Barbara