Look at the differences between the queries Google provide for IE and FF
http://www.google.com/search?hl=en&q=animated+websites
vs
http://www.google.com/search?hl=en&q...=Google+Search
at both cases we are on the same position #4, but queries are different!
My guess, taking into account the fact that Google owns domain - gbrowser.com and rumours about Google wish to brand
FireFox as its own browser....I assume they do make difference for IE and FF queries.