Obtrusive CSS calling on Firefox Extension

We have an issue with a css call in the Avast! Firefox Extension with Firefox 26.

The file : chrome://wrc/content/common/skin/css/extension.css is called on every page during the navigation. There is a “.i” css class in this file with some “!important” css property. Problem : we use the same class for data table in our websites. Since the Firefox 26 update a lot of our data tables are broken by this intrusive css.

Why is this CSS file called on every page on Firefox?