Recently (maybe 4 weeks ago), the behavior on my system changed so that now when I click links in email messages in Thunderbird, I get a GNOME notification that “Firefox is ready”, but the focus does not change to Firefox as it used to. If I Alt-Tab or otherwise switch to the Firefox window, the tab which was opened for the link is visible as expected, but I need to manually switch to it.
If you search for ready on https://extensions.gnome.org a few extensions that change this behavior are available. I can’t speak to how well they work as I haven’t used them.