There is no problem with Thunderbird as everything is working correctly.

However, the issue seems to be in Mozilla Firefox browser, ideally this question should be in the 'Firefox' help forum. But try this when Firefox browser is opened. Menu app icon > More Tools > Customise Toolbar A new tab opens - at the bottom you will see 'Title bar' and next to it 'Toolbars' click on the drop down and select 'Menu Bar'.

If the problem is with Firefox why does it only occur with Thunderbird? Every other link runs fine in Firefox. If I past and copy the link(from Tbird) into Firefox I get the top line. It is only when I click on a link in Tbird that I get no top line in Firefox.

Are you saying that you clicked on a link and it opened in Firefox, you then followed my instructions to enable the 'Menu Bar'. Then closed Firefox. Then clicked on the link again and the Menu Bar was still not showing, despite setting it previously?

the answer to your question is YES. Note that firefox loads with the menu bar with every link EXCEPT the links in Thunderbird.

the answer to your question is YES. Note that firefox loads with the menu bar with every link EXCEPT the links in Thunderbird.

So if Firefox is open, does a new tab open and the menu is turned off? Or does a new instance of Firefox open without the menu bar?

a new Firefox window.

Sounds like the profile Firefox is using for your opened from links and open from elsewhere is different.

Open the Firefox menu and in options set it to open links in a new tab instead of a new windows and see if that helps.

Otherwise while you have both windows open, type about:profiles into the location bar of each and check that the "profile is in use" profile shown is in the same location on both windows.