There are many reports of this problem and some possible fixes.
See these links:
http://forums.silverlight.net/p/227806/614938.aspx
"I decided to try uninstalling and reinstalling firefox, and that did the trick."
https://support.mozilla.org/en-US/questions/795168
"Enabling plugin-container.exe in about: config (dom.ipc.plugins.enabled) fixes the mouse problem, but then the Silverlight plug-in pop-ups (like webcam / microphone resolution) freeze the browser."
Windowless Silverlight (and Flash?) In Firefox: Incorrect Mouse Messages
"This problem does not occur if you do one of the following:
- Switch Firefox to full screen mode (it seems that the top panel is causing the bias, so when it is not visible, there is no bias).
- Do not use Silverlight in windowless mode.
- Use any other browser.
...
In my research, I came across the following suggested workarounds ...:
- Do not use overflow in css.
- Use css to set the 1px html field.
- Do not use iframe "
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