A possibility is to create a plugins folder in the Firefox installation folder and place a symlink to the Flash plugin file libflashplayer.so in that folder.

See also:

• http://kb.mozillazine.org/Determining_plugin_directory_on_Linux

Sorry, but that doesn't change anything on my laptop, and there's no libnullplugin.so. Did I install firefox correctly? I downloaded the linux version (4.0 for linux i686) and just extracted the files in a directory, and I start it by using the file named firefox in the main directory (waiting for the moment when firefox 4 is available for update in ubuntu). There's no plugins directory (and even if i create it and put in the libflashplayer.so (also, 64-bits version), that doesn't change anything). My previous version of firefox (3.6) installed in native works nevertheless fine. Thank you for your answer.

Firefox 4 doesn't have the libnullplugin.so anymore and that made the plugins folder empty. Therefor that plugins folder no longer exists by default and needs to be created if you need or want to install via the plugins folder. Plugins placed in that folder are still recognized and used.

My firefox doesn't seem to detect the plugin, though I have created the folder "plugins" in the proper location (the main folder)... I don't understand...

Are both the Firefox program and the Flash plugin 32 bit versions (or 64 bit versions) ?

That's it! Effectively, the firefox 4.0 I downloaded is 32 bits (I thought not because of the "i686" name). And the native firefox 3.6 was a 64 bits programm (I didn't know either). That's why I thought I was doing things correctly when I tried to install the 64 bits plugin. So it works with the 32 bits plugin. Thank you !