Silverlight only supports non-64 bit versions of Firefox on OSX (and from Firefox 4 onwards we're shipping 64-bit Firefox to 64-bit OSX users) and unlike with other plugins, we can't just start a 32-bit process to run it because of issues with the silverlight plugin architecture. We're working with the Silverlight team at Microsoft to get those fixed but it's essentially waiting on them.

(Or you could try to pressure them into releasing a 64-bit Silverlight for Macs.)

Thank you Cww, ...

I hope MS release 64-bit version of Silverlight ASAP.

In the meantime, you could try running Firefox 4 Beta on Mac OS 10.6 in 32-bit mode (this worked for me):

1. From the menu bar, select Firefox and then select Quit Firefox
2. Open the Applications folder in Finder and locate the Firefox application icon.
3. Hold down the control key while you click the Firefox icon and select Get Info from the list of menu items.
4. In the "Firefox Info" window, click the box in front of "Open in 32-bit mode" so that it is checked.
5. Close the "Firefox Info" window and then restart Firefox 4.

(The above steps were copied from another article that also suggests running Firefox 4 Beta in 32-bit mode as a workaround for a different plugin issue.)

P.S. Here's the related bug report, in case you want to vote for it or keep track of its progress:

https://bugzilla.mozilla.org/show_bug.cgi?id=598406
Bug 598406 - Silverlight not supported by i386/x86_64 universal binary on Mac OS X 10.6

thank so much

I really haven't bumped into many sites using silverlight.. Can you point me to a site using it so i can check if mine works..