Links in received messages normally open with a single click if they are underlined blue text. But some that appear as plain text do not, due to a malformed format in the sender's message, especially when sent from mobile mail apps.

If you are double-clicking links in composed messages, they do not open by design.

Thank you for the response. The links don't open with any type of click, even if they are underlined. I click and or double-click and nothing happens. Since I asked this question, I also have found that other programs (ie. LibreOffice and more) will not open links in Firefox, so it might not be Thunderbird only that is the problem.

I suggest you open Windows' 'default apps' app and check what links Firefox is set to acknowledge.

Thank you, David. This seems to be all correct (htm, html, http, https, etc.) - Firefox is set for all of these.

Try reinstalling Firefox, choose Advanced setup, set FF as the default web browser, do not run at the end of the install, restart Windows (important), then see if links in messages open in FF.

Similar topic: https://support.mozilla.org/en-US/questions/1307664

Thank you, @sfhowes! I didn't even have to do all of this. When I went to install FF again, it asked me if I wanted to "refresh" intead and the "refresh fixed the problem. I appreciate your help!