Why, when I click a web link in Thunderbird, does the Unity launcher open a second instance of the browser?

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http://askubuntu.com – If I click on Chromium (which I've set as my default browser) in my Unity launcher (in 12.04), then it opens as expected. However, if I click on a URL in a Thunderbird email, then Unity opens a second instance of Chromium, inserting another Chromium icon into the Unity launcher. I haven't tried this with Firefox, but I don't think I need to, since my preferred browser is Chromium. Is this desirable behaviour? EDIT: Just noticed that this also occurs when I click on a link in an Empathy chat conversation. (HowTos)