[Users] Display of mail attachment in running browser

[Haines Brown]

My wife runs claws mail on debian. When she opens a message with an
attachment, a URL is displayed. When she clicks on the URL to view the
attachment, an error message pops up that Firefox is already running and
must be closed. However, she runs virtual desktops in which claws mail and
firefox are always running. 

Is there any way to have claws send the URL to a running instance of
Firefox rather than having to stop the Firefox application and starting it
afresh for each HTML mail attachment?

Haines Brown