2 links in same TBird email open differently in Firefox. How 2 make them behave the same?

I run Tbird 91.2.0/Linux & Firefox 94.0/Linux

In Firefox’s user.js I’ve set

	user_pref("browser.link.open_newwindow", 3);
	user_pref("browser.link.open_newwindow.restriction", 0);
	user_pref("browser.link.open_newwindow.override.external", -1);

IIUC, with that config, links opened from other apps should open in new tabs in an existing/running instance of FF.

In a Tbird email, I’ve two links.

If I click on the 1st,


it behaves in exactly that way … opened in a new tab in existing FF instance.

But if I click on the 2nd,


a new FF instance is launched, and the link is opened in a new, sized window in that new instance.

Why do the two TBird links behave differently with this config?

What do I need to change so that ALL Tbird links behave like the 1st one – opened in new tab in the existing/running instance of FF?