From MozillaZine Knowledge Base
about:plugins displays a list of installed and detected plugins, including the file name, version, and associated MIME types. To view this list:
- Firefox: Type about:plugins in the Location Bar (address bar) and press the Enter key.
- SeaMonkey: Open the Help menu and choose "About Plug-ins".
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 "Enabled" or "Disabled" for each plugin (bug 831533).
Plugin file location:
- Mozilla 20 and below: To show the full path to the plugin instead of just the file name (e.g., in Firefox 20), type about:config in the address bar and press the Enter key. Find the preference plugin.expose_full_path and set it to true (double-clicking the preference name will toggle the setting). Caution: Exposing the plugin location may have security implications on some systems. It's best to reset the preference to the default "false" value when you are done. 
- Mozilla 21: There is no way to display the full file path in the about:plugins list. An alternate way to find the plugin file location (e.g., in Firefox 21) is to inspect the contents of the "pluginreg.dat" file, located in the profile folder for your Mozilla application.
- Mozilla 22 and above: Starting in version 22 (e.g., Firefox 22) an extra row "Path:" is included in the about:plugins list, which displays the full file path for all plugins. (bug 661961).
- This screenshot shows the beginning portion of an about:plugins list in Firefox 25 on a Windows 7 64-bit system:
In some cases, about:plugins may display outdated information, even after a restart. To refresh the list, open the profile folder, then close your Mozilla application (if open) and delete (or rename) the file "pluginreg.dat". The file will be generated when you restart your Mozilla application; however, any plugins disabled in the Add-ons Manager will be re-enabled.  
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