Created attachment 8895854 [details] testcase.xpi I noticed the problem from https://github.com/Robbendebiene/Gesturefy/pull/18#issuecomment-321553312. STR: 1. Load the testcase.xpi in about:debugging#addons. 2. Click the added toolbar-button, check the popup, the second line is garbled. Actual results: Unicode characters broken, regardless of the css or html encoding. Expected results: Correctly handle characters.