31 fps is used 'cause historically it was one of the few frame rates that the mac Flash player could cope with ( Setting a fps of 30 would result in 19 fps on the mac ).
Just to cover it to death, it was a case of 31-58 fps on the mac would run at 29fps, so 31 became the norm as a happy medium.

It's probably not been valid since F6, but it's a habit thing for a lot of developers.

Pushing the fps artificiality high isn't the way to go to get the best playback. It means the screen is updated more often, which can take cpu cycles from the actual code.

wmode does give a performance increase, but in the case of FireFox it stops the arrows keys ( Amongst others I believe ) from being registered in Flash.

Squize.