Discourse modification bug in the title

On my firefox something wierd is happening with this forum software, the way it was customized for Patreon community.

This is not happening on Chrome/Chromium. This is on Firefox 54.0 64bit on Ubuntu Linux.

I have no problems with other sites using the same (Discourse) forum software (for example: https://llllllll.co/ or https://discourse.joinmastodon.org/ )