|As it happens the self same thing happens in my Firefox although I ignored it. I think this is the fix:|
"Go into add-ons, click plugins on left, and disable "Silverlight plugin". You will probably still have to click the bookmarks one by one and open each website to make its icon return.
I also found that when you had restored some icons, after re-enabling Silverlight you could still make the remaining missing ones appear by going to their websites.
Rather strange all this but it works - seems like a bug in the addon.
Always pop back and let us know the outcome - thanks