[I] Broken icons in the extension for firefox

[fabianski7]

Actual behaviour

Using some modification script for firefox, such as ghacks/user.js (to improve security and privacy of the browser), causes the icons not to be displayed and the extension to get messy.

Screenshot

Steps to reproduce

1. Open the advanced firefox preferences (about:config) and change the browser.display.use_document_fonts option from 1 to 0
2. Disable and enable the extension

More information

here are some cases I found with more information about this bug and how it was fixed.

CTemplar/webclient#508
arkenfox/user.js#824
FirefoxBar/HeaderEditor#46

[pbek]

Can you please open this at https://github.com/qownnotes/web-companion/issues