Currently, the chessboard is not compatible with a certain Firefox privacy setting: privacy.resistFingerprinting
This setting is disabled by default but might be enabled when installing Firefox privacy extensions.
It can be disabled again like so:
Go to about: config
In the search field paste in the name of the setting: privacy.resistFingerprinting
Double-click "privacy.resistFingerprinting" to set its value to false
This should resolve your problem - If not, please get in touch with support.