From MozillaZine Knowledge Base
On Windows systems, Mozilla applications will scan for common plugins at startup, such as Java, Flash, QuickTime and others, in locations external to the installation directory plugins folder (which no longer exists, by default). This article identifies the related preferences that control this behavior and explains how these settings can be modified.
Disabling the plugin scan
To return to the default settings, reset the modified preferences via about:config. Note that these changes will affect the current profile and will need to be repeated for any other user profiles.
Starting in Firefox 3 and SeaMonkey 2, you can disable individual plugins via "Tools -> Add-ons (Add-ons Manager) -> Plugins", by selecting the plugin and clicking "Disable" as shown here. Since there is no separate plugin.scan preference for each plugin detected via the Windows registry, the Add-ons Manager provides an easy way to manage these plugins.
For example, if you have Google software such as Google Chrome or Google Earth installed, the Google Update plugin is also installed. The Google Update plugin is then detected in your Mozilla browser via plugin scanning because its directory is specified in the Windows registry for PLIDs. You can disable the Google Update plugin using the Add-ons Manager.  
Note: In Mozilla 20 and below (e.g., Firefox 20), disabled plugins do not appear in the about:plugins list. Enabling a plugin via "(Tools) -> Add-ons (or Add-ons Manager) -> Plugins -> Enable" will add it back to the list. Starting in version 21, all plugins will be listed and about:plugins will show either "Status: Enabled" or "Status: Disabled" for each plugin (bug 831533).
Plugin scanning is controlled by the preferences listed below, which is presented for information only. Although it is possible to stop the plugin scan for for all user profiles by editing the "all.js" file, as mentioned here, this is not recommended since modifications made to all.js will be lost at the next program upgrade that replaces the file with a newer version.
Any line preceded by a double slash ("//") is a "comment"; the actual preference lines begin with
In versions since Firefox 0.9 and Mozilla Suite 1.7.a  these preference settings are contained in the "all.js" file, located in the <installation directory>\greprefs\ subfolder in Firefox and in SeaMonkey 1.1 or above (e.g., C:\Program Files\Mozilla Firefox\greprefs\all.js) or in the GRE\<buildID>\greprefs\ subfolder in SeaMonkey 1.0.x and Mozilla Suite (e.g., C:\Program Files\Common Files\mozilla.org\GRE\22.214.171.124_2006091003\greprefs\all.js).
// Locate Java by scanning the Sun JRE installation directory with a minimum version // Note: Does not scan if security.enable_java is not true pref("plugin.scan.SunJRE", "1.3");
// Locate plugins by scanning the Adobe Acrobat installation directory with a minimum version pref("plugin.scan.Acrobat", "5.0");
// Locate plugins by scanning the Quicktime installation directory with a minimum version pref("plugin.scan.Quicktime", "5.0");
// Locate and scan the Window Media Player installation directory for plugins with a minimum version pref("plugin.scan.WindowsMediaPlayer", "7.0");
// Locate plugins by the directories specified in the Windows registry for PLIDs // Which is currently HKLM\Software\MozillaPlugins\xxxPLIDxxx\Path pref("plugin.scan.plid.all", true);
// Controls the scanning of the Navigator 4.x directory for plugins // When pref is missing, the default is to pickup popular plugins such as // Flash, Shockwave, Acrobat, and Quicktime. If set to true, ALL plugins // will be picked up and if set to false the scan will not happen at all //pref("plugin.scan.4xPluginFolder", false);
Note that the last preference line listed above is "commented out" by the double slashes ("//"), causing the preference to be missing, which enables the Netscape Navigator 4.x plugin scan.
Related bug reports