why does thunderbird icon in panel launch wrong binary?

I downloaded a new version of Thunderbird (13.0.1) and extracted it into my ~/bin directory.

I get rid of the old Thunderbird icon in the launcher on the left of the screen by right clicking and selecting 'unlock from launcher'.

I run my new version in a terminal like this :

$ ~/bin/thunderbird/thunderbird > /dev/null 2>&1 &

and see that an appropriate Thunderbird icon appears at the bottom of the launcher, so I right click it and select 'lock to launcher'.

If I then quit Thunderbird and use the new launcher icon to launch it, it launches the old version of Thunderbird.

Why is that, and how can I make it launch the new version? In previous versions of Ubuntu, I could edit each icon's 'preferences' and point it at new versions, but there isn't that option now, so I don't know how to fix this.

12.04