Create a [http://developer.mozilla.org/en/docs/Plugins plugin] test suite. There are currently no tests of any kind, and this code is very brittle and not well understood. This would involve creating a plugin or set of plugins and some html/js to drive that plugin in order to push test coverage. Maybe look at the JRE tests, create some specific plugin cases (e.g., swf to load that tests flash specific code). The newly announced Google Chromium browser [http://dev.chromium.org/developers/how-tos/getting-around-the-chrome-source-code seems to have something like this] with regard to using the npapi for driving tests via plug-in.