I have also been experiencing this but installing the ppa:sevenmachines/flash has fixed it for me.
flashplugin64-installer Version: 10.3.162.29-0ubuntu0~sevenmachines2
with firefox 5.0+build1+nobinonly-0ubuntu0.10.10.1~mfs1
on Ubuntu 10.10 x86_64
I have also been experiencing this but installing the ppa:sevenmachin es/flash has fixed it for me.
flashplugin64- installer 29-0ubuntu0~ sevenmachines2
Version: 10.3.162.
with firefox 5.0+build1+ nobinonly- 0ubuntu0. 10.10.1~ mfs1
on Ubuntu 10.10 x86_64