[ Thanks to Martin for this link. ]
“When you click on a link in an email it will open up
in Thunderbird “browser” thus cutting down the time it
takes to open a link from an email in your browser. Each link you
click-on can open up on a new tab in Thunderbird if you configure
it that way, so in some way ThunderBrowse also brings tabs to
Thunderbird 2.“ThunderBrowse is also highly customizable. You might prefer
opening up links in an alternate browser, no big deal, that can be
done in the External Browser tab in ThunderBrowse Options. You can
quickly open a new tab or window and scroll through the
“browsers” history with Hot keys. A history-clearing
button is found in the options menu.”