Quicktime Mime Settings

Hi,

I have been having troubles with Quicktime and streaming movie files (Mov i assume).

the main mov streaming movies i watch are on the apple trailer site.
i get the infamous faded Q with a white questionmark.
i am using FireFox 1.5.0.7 with Quicktime 7.1.3
i have checked Aboutlugins and it shows Quicktime to be installed correctly.
i have noticed that when i go into hte Quicktime Mime Settings and go to Miscellaneous and Quicktime HTML is unticked. if i tick it with Firefox closed, and then close the prefpane i can usually watch maybe 1 trailer and then the check box in the mime settings will become unticked. and i have to repeat the whole process (Close firefox > Tick QHTML > Watch Trailer > Repeat).

i have tried everything, google, searched this forum and a number of others and i could find no answers for my problem. any help would be greatly appreciated.