Sorry, text should be not pasted. It shoul be dragg-and-dropped onto the textbox.
I tried to attach movie showing how to recreate the issue but movies are not allowed as attachments.
II will try to explain again all steps you must do:
1 Open Firefox 3.6.3
2 go to http://demos.telerik.com/aspnet-ajax/input/examples/common/orderform/defaultcs.aspx
3 select some text i.e. 'The following example'
4 drag text onto Billing Address textbox
5 text is pasted
6 click on the price textbox (or somewhere else)
7 click on the Billing Address textbox again
8 text pasted dissapeared
8a) when you type some text in textbox and then drag-and-drop other text, only drag-and-dropped text dissapear