According to the info in your forum post, you've got "Shockwave Flash 9.0 r124".

If you go to Tools > Add-ons > Plugins what version of Flash does it say you have?

In any case, try using the instructions at Flash Plugin - Keep it up to date and troubleshoot problems

As I said before: I already did install the Adobe flash version 10 (Debian). It "installed succesfully".

Proof of succesful installation (see attached screen shot)

partial about:plugins output: Shockwave Flash

   Bestand: /home/jules/.mozilla/plugins/libflashplayer.so
   Versie: 
   Shockwave Flash 9.0 r124

MIME-type Beschrijving Achtervoegsels Actief application/x-shockwave-flash Shockwave Flash swf Ja application/futuresplash FutureSplash Player spl Ja

So this looks like an issue of making Firefox detect the new version of Flash on your computer. I'm not a Debian user, but maybe there is someone else here who can help.

Remove the current Flash player from /home/jules/.mozilla/plugins/libflashplayer.so If that is a link then you may have to remove (uninstall) that old Flash Player as well.

Create a symbolic link to the newly installed Flash 10.0 r45 plugin in the /home/jules/.mozilla/plugins/ directory to make Firefox find it.

@Cor-el I first renamed the file /home/jules/.mozilla/plugins/libflashplayer.so to /home/jules/.mozilla/plugings/libflashplayer9.so and created the link to the /usr/lib/adobe-flashplugin/libflashplugin.so In firefox about:plugins just detected the renamed file and still reported flash version 9 Then I deleted the file libflashplugin9.so In firefox about:plugins now reports the file /usr/lib/adobe-flashplugins/libflashplugin.so to be flash version 9 !!

Try to delete the file pluginreg.dat in the Firefox Profile Folder and let Firefox rebuild a new version.

See http://kb.mozillazine.org/Issues_related_to_plugins