I hate Flash. But that is a rant for a different post.
I don't have Flash installed, and I don't want to have Flash installed. That means I get Firefox's yellow "Additional plugins are required to display all the media on this page" informational bar on many pages with Flash content. Here are instructions for disabling it in Firefox on Ubuntu 8.10.
[Update: much easier way to do this, apparently available on Firefox 3.0+: in Firefox, simply go to about:config and set plugins.hide_infobar_for_missing_plugin to true. Thanks, Rakhun!
The more complicated way I originally suggested doing this follows.]
In Firefox, go to about:config and set plugin.default_plugin_disabled to false. By itself, this suppresses the yellow plugin bar and replaces it with a dialog, which is even worse! In order to disable this dialog, you'll also need to delete the file /usr/lib/xulrunner-addons/plugins/libnullplugin.so.
(The "null plugin" or "default plugin" is responsible for rendering a blank box and prompting you whenever Firefox encounters content it doesn't have another plugin for.)
If you ever regret this decision, you could keep libnullplugin.so somewhere safe and just move it back to that directory—or, just reinstall xulrunner-1.9 with aptitude reinstall xulrunner-1.9.
On other systems, you can identify the location of the default plugin by setting plugin.expose_full_path to true. Then go to about:plugins and look for the "Default plugin". Firefox should show you the path to the plugin file. Delete that file.