The Silverlight.js script provided by Microsoft for embedding Silverlight objects in webpages calls navigator.plugins.refresh, causing main-thread I/O. Silverlight.js documentation: http://msdn.microsoft.com/en-us/library/cc838126%28v=vs.95%29.aspx Can we be smarter about caching this plugin data to avoid doing some of this I/O?

Well, it depends on what you mean. navigator.plugins.refresh() is the API for discarding the cache and searching again. We could certainly make it asynchronous, at some risk of minor breakage. We should probably focus our effort on how long this takes at startup. See also bug 959224.

Aside from startup, navigator.plugins.refresh() is used exclusively by pages directing a user to update a plugin, which hopefully is not happening often. I would be more concerned about sites calling refresh() superfluously (to make sure the plugins are fresh!) We neutered it partially in bug 418615 and saw no fallout, it's probably not unreasonable to make it async and risk requiring updates on the half-dozen legitimate pages using it.