After following my own instructions, I ended up with a Flash instalation that could play YouTube videos correctly, but some other Flash animations would not work. By chance, my computer at work would reproduce any Flash animation just fine, so... why would that be?
To find out the reason, I have compared what Flash-related packages I have installed in Homer (my computer at work) and Heracles (the one at home). The result is quite surprising:
Homer[~]: aptitude search flash p flashplayer-mozilla - Macromedia Flash Player p flashrom - Universal flash programming utility p flashybrid - automates use of a flash disk as the root filesystem p libflash-dev - GPL Flash (SWF) Library - development files p libflash-mozplugin - GPL Flash (SWF) Library - Mozilla-compatible plugin p libflash-swfplayer - GPL Flash (SWF) Library - stand-alone player p libflash0c2 - GPL Flash (SWF) Library - shared library p libroxen-flash2 - Flash2 module for the Roxen Challenger web server p m16c-flash - Flash programmer for Renesas M16C and R8C microcontrollers p vrflash - tool to flash kernels and romdisks to Agenda VR Homer[~]: aptitude search swf p libflash-swfplayer - GPL Flash (SWF) Library - stand-alone player p libswf-perl - Ming (SWF) module for Perl p libswfdec-0.5-4 - SWF (Macromedia Flash) decoder library p libswfdec-0.5-4-dbg - SWF (Macromedia Flash) decoder library p libswfdec-0.5-dev - SWF (Macromedia Flash) decoder library v libswfdec-dev - p pyvnc2swf - screen recording tool to SWF movie v swf-player - p swfdec-mozilla - Mozilla plugin for SWF files (Macromedia Flash) p swfmill - xml2swf and swf2xml processor
Yes, Flash works perfectly at Homer because it has no package installed with swf or flash in their name! And I don't have any Gnash package installed, either. I removed all swf/flash-related packages on Heracles, and now Flash works perfectly in my home computer too.