Rated 4 out of 5 stars

When I close my browser, I get the following error message: TypeError: utils.getPref is not a function
Can Anybody tell me what this error means in english?
Other than that, great Add-on.

This review is for a previous version of the add-on (1.48.2). 

Please try the latest version 1.48.3

from here: https://addons.mozilla.org/en-US/firefox/addon/6623/versions/

It is not reviewed yet by AMO, but fixes a few bugs of the current version including possible namespace conflicts with other addons (which seems to happen in your case)