Тоже возникла некоторое время назад такая ошибка. Скачал tar.gz с оф сайта, установка описана в readme, процитирую:
Installing using the plugin tar.gz:
o Unpack the plugin tar.gz and copy the files to the appropriate location.
o Save the plugin tar.gz locally and note the location the file was saved to.
o Launch terminal and change directories to the location the file was saved to.
o Unpack the tar.gz file. Once unpacked you will see the following:
+ libflashplayer.so
+ /usr
o Identify the location of the browser plugins directory, based on your Linux distribution and Firefox version
o Copy libflashplayer.so to the appropriate browser plugins directory. At the prompt type:
+ cp libflashlayer.so <BrowserPluginsLocation>
o Copy the Flash Player Local Settings configurations files to the /usr directory. At the prompt type:
+ sudo cp -r usr/* /usr
После этого перезапускаем файрфокс - вуаля
