Same experience as user 15218768. This add-on (v6.2.0) creates massive loss of performance (major lags) on all Google-related pages in FireFox.

The firefox version of this extension lacks certain crucial features that make it rather useless. For example, one basic function that it lacks is the ability to save pdf files directly to my library from firefox's in-built pdf viewer. Instead, the extension merely saves the url to my library. I would encourage the developers to add the same functionalities to the firefox extension as those on the chrome extension.