I was having the same issue with RadTextBox Multiline in FF and found this post.
After some tests I can say that the problem arises when the MaxLength property is used!
My original textbox which was causing the problem Kuba described:
|<telerik:RadTextBox ID="RadTextBox1" runat="server" TextMode="MultiLine" Rows="5" Columns="5"
| TabIndex="4" MaxLength="500" Height="75px" Width="325px">
After I removed the MaxLength, it behaved fine!
I confirmed this behaviour using the Billing Address radtextbox from the online demo
provided by Sebastian in my web app.
It was working correctly but, as soon as I added a MaxLength property, the scrolling didn't work anymore as supposed.
Can you guys at Telerik investigate and provide a fix for this?
Ps: I just checked in IE8 and had no problems