I am going crazy over this, but clicking https links in Thunderbird won’t switch focus to the browser (in my case Firefox). The link opens in a new tab in the browser, but the focus stays on TB. Searching the web for similar issues, most questions seem to address the opposite problem where the user don’t want focus to switch away from TB.
I fail to find a preference setting in either TB or FF where I can chose the behaviour of https links in TB.