If found Firefox and Flash on Ubuntu isn't as easy as other browsers
such as Opera / Chrome. With these other browsers all you need to do
is install the Ubuntu Restricted Extras packages from the Ubuntu
Software Centre and it just works ! Firefox has the need to have the
plugin installed separately and it doesn't always take - made more
problematic if you have the 64bit version of Ubuntu :-(

What you can try is totally removing Flash using Synaptic Package
Manager then installing it again and/or installing the swfdec package
which is available in both USC and Synaptic.

FWIW - I've found Opera less hassle and only have Firefox for the few
sites that don't like Opera ( amusing anecdote - Placido Domingo's web
site doesn't like Opera - or at least it didn't the last time I went
there ).