Speed dial not loading images #564

Hello!
I have a problem with my speed dial extension when user.js is enabled. On every launch this extension downloads logo image from every site listed in it and displays this logo in the visual cell for this site. Also it gives me the ability to save logos for each site, and if I do, the extension stops downloading logos and starts displaying them from storage. But when user.js is enabled, it doesn't display a single logo - neither downloaded, nor saved.
The console for the extension page shows the following message:

Source map error: NetworkError when attempting to fetch resource.
Resource URL: moz-extension://********-****-****-****-************/js/lib/browser-polyfill.min.js
Source Map URL: browser-polyfill.min.js.map

What settings in user.js could cause this failure?