Over the last number of days I noticed my Firefox (on Mac) was getting increasingly unstable. It was freezing completely after being open for random amounts of time (but generally not very long!).
I had recently installed a couple of new Firefox extensions so I was blaming them. However, uninstalling them made no difference.
Today I think I found the culprit. I had about 40 tabs open in Firefox (in three separate windows). I scanned through the tabs and closed all the tabs containing Silverlight content and since then, Firefox has been rock-solid.
Looks like there is still a ways to go with Silverlight and Firefox on the Mac.