I thought Thunderbird would open a web page as a tab within Thunderbird, am I wrong? If not then something is wrong with my setup as this isn't my experience. A web page link always attempts to open directly in Firefox, outside of Thunderbird. I use attempts because of the explanation below.

When I click a link within an email, a new Firefox tab is created displaying the web page, however, if Firefox wasn't already open then it is opened but it is completely transparent and functionally unusable. By mousing around the screen I'm able able to find it (hand pointer icon is displayed) and drag it to another desktop. Then starting Firefox manually in the first desktop I can enter the link and see the site. Firefox never becomes visible on the second desktop.