[imp] Weird behaviour with firefox and checkboxes on IMP 4.1.3

[Fabio]

This started to happen when I upgraded to IMP 4.1.3.
When there's a message list being displayed, pressing <shift> while 
clicking on the checkboxes no longer lets you select multiple messages 
in a first->last fashion (select the first, press shift, select the last 
and then all messages in the range get selected).
Also, if you press <shift> and double-click a checkbox, it *disappears*!!
It happens with Mozilla Firefox 1.5.0.6; works normally with IE6/win and 
Opera 8.5.
What's up with that?