Fix? Were you using the now non functional thunderbrowse add-on in the past perhaps? Normally all links open in the systems default browser, except for some specific internal links like the Thunderbird welcome link.

It's not Tbird It's Firefox 67 update

In Firefox, type about:config in the address bar and copy browser.link.open_newwindow.override.external into the search box. The default value is -1, but if it's any other value, right-click, Reset.

https://support.mozilla.org/en-US/questions/1193456#answer-1046550