Is it possible to make flash player download the full content before playing
when playing streaming movies? My connection speed is pretty slow and when
there's a flash, FP will play it right away, making it jittery. The only way I
can enjoy this is by muting the volume and let it download the whole movie.
Then I'll get back to it when it's done. It works but I really don't want to
bother with muting the volume.

Currently no, there is not.

Take a look at our new Adobe Media Player, currently in beta:
http://labs.adobe.com/wiki/index.php/Media_Player