• In the Location bar, type about:config and press Enter .
• The about:config "This might void your warranty!" warning page may appear.
• Click I'll be careful, I promise! , to continue to the about:config page.
• In the Filter textbox, enter plugins.hide_infobar_for_missing_plugin
• Right click it and select Toggle so the value changes to true
• You might need to restart for this change to take effect.

Additional problems:

Your Adobe PDF plugin (Adobe Acrobat Plug-In Version 7.00 for Netscape) is dangerously out of date.

This old version in particular is a common cause of crashes.

Run a Plugin Check at http://www.mozilla.com/en-US/plugincheck/ and get it up to date.

Thanks for the help but what you suggested had no effect, sorry.

That's all I have to offer.

This procedure works for me with Firefox 3.6.4 and http://www.vdat.com/techsupport/windowstest.asp (without the WMP plugin)

See http://kb.mozillazine.org/Disabling_yellow_plugin_bar_-_Firefox