i'm not sure if this is only happening with forms but thats where i notice it all the time.
for example at http://extjs.com/deploy/ext/examples/form/dynamic.html
scroll to the form "Multi-column, nesting and fieldsets", there is a textfield labeled "address" with the sample text "4 Redbulls Drive"
then single click on address then move the mouse over the text in the textarea...
the text gets selected as if the mouse button was held down.
(only happens with FF!)