From MozillaZine Knowledge Base

about:plugins is a feature of Mozilla browsers that displays a list of installed plugins, including the file name and associated MIME types. In addition to plugins found in the installation directory plugins folder, it also lists plugins installed in other locations that are detected via plugin scanning.

To display a list of installed plugins:

To show the full path to the plugin instead of just the file name, type about:config in the address bar and press the Enter key. Find the preference plugin.expose_full_path and change the value to true (double-clicking the preference name will toggle the setting).

This screenshot shows a Firefox about:plugins list on Windows: