When using <F11> with Exposé on Panther Firefox windows are hidden whereas dropdown lists (location bar or <select>) remain displayed.
WFM on Jaguar (10.3.5) with FF/0.10. So I guess this is Panther-specific.
This bug has been buried in the graveyard and has not been updated in over 5 years. It is probably safe to assume that it will never be fixed, so resolving as WONTFIX. [Mass-change filter: graveyard-wontfix-2014-09-24]