I'm using Firefox 1.0 preview but I had the same problem with both .8 and .9
If I click on a link/image that requires flash, I get the message that Flash has to be installed. I install it and specify all the relevant browsers and Firefox still doesn't recognize that it's there. I tried this several times with no luck.
I have the same problem with Mozilla 1.5, but it works fine with IE, Netscape and Opera.