Please Help! - I can't watch flash movies!

Hello,

my standard browser is Mozilla Firefox version 1.5.0.6. I just installed the
new version of Quick Time Player (7.1). Now it's not possible to watch flash
movies with Firefox. Everytime I click on the movie icon to start the stream,
there only appears the Quick TIme Logo with a questionmark. So obviously Quick
Time tries to play the Flash movie but is not able to do so. I don't know what
to do and where to change any options. With Internet Explorer I still can watch
the Flash movie without any problems.

Does anybody has an idea what I can do?

Thanks a lot!