I had already tried any of those before posting, without success.
Actually, synaptic installer now shows the Adobe flashplayer as "installed", but my Mozilla Firefox doesn't agree!
Maybe I should paste manually the libflashplugin.so in somedirectory of firefox?
Maybe the problem is my installation, looking in the usr/lib directories I see that I have installed both firefox 3.0 and 3.5, and the icon on my desktop actually launches the 3.0 (strange! I made only automatic update..).
However, I tried running the 3.5 manually, but the result is the same, my firefox doesn't see the Adobe flashplugin, even if according to synaptic it's there!