How to read a registry-entry??

[swarovski]

use read registry.
type
READ REGISTRY in the MSICode view.

like this:
Read Registry Key HKLM\\SOFTWARE\\Mozilla\\Mozilla Firefox 2.0.0.9\\extensions\\Plugins into returnValue

[SimonL]

You are better off using the MozillaPlugins Registry key to record where your plugin resides, then it will be available to all Mozilla installations and you can choose where to install the plugin for yourself.

The User's Local Application Directory is one place to put it.