I noticed this a while back when U.S. Airways and America West merged. The new website had the U.S. Airways logo icon, but since I originally moved (drag/drop)the website into the bookmark toolbar, it kept the America West Icon which I like better.
There was a text messaging site that no longer seems to message my cell phone anymore, so I found the link on T-Mobile to send messages to my phone. I wanted to keep the icon used with the original messaging service since I already had a T-Mobile icon on the bookmark to the link to my account and the icon for the messaging website had a more appropriate icon for text messaging. I noticed that if you just change the link address through "properties" it will keep the old icon even though the new website has it's own.
Is this just a glitch in my firefox (2.0 now .12) or can the rest of you do this? Also, is there a way to attach any icon to a site you want to a bookmark?