My laptop is an ibook G4 and maybe because of madness or because I'm so geek, I use linux on my ibook. One of the most annoying problems is that there is no good support for flash in linux PPC. This causes that all webpages viewed with firefox that requires the flash player plugins makes firefox requesting to install it.
After being annoyed for months I search for this configuration in about:config page and I finally found it. The only action to avoid this message is to set plugin.default_plugin_disabled to false and that's all!. Now I can surf the web without any Install missing plugins message.