I first reported this problem in February 2020 and still have no solution. I am now using Firefox version 80.0. When I click on an external link, like a link embedded in the body of an email, two browser windows open. One window is the homepage and the second is the URL of the external link. This only happens if the Firefox browser is not already opened. If the browser is already opened, the external link URL opens in a second tab as expected. I've already followed directions to change the action for a content type and reset actions for all content types in article https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows but it did not fix the issue.