Toolbar icon disappears in HiDPI mode

[cdfmr]

Your DownThemAll! and Browser Version and Language and Operating System

DownThemAll 3.0.8
Firefox 56.0.2
macOS 10.12.6 / Windows 10

Expected Behavior

Actual Behavior

Under HiDPI mode, the toolbar icon of DownThemAll disappears, only a transparent frame when mouse cursor moves over the button.

Steps To Reproduce

Additional info such as log extracts

Please enable Diagnostic Logging in the Privacy tab of the DownThemAll! preferences and either copy/paste essential part of the log or attach the entire log. Please note that the log may include sentitive information which you have to remove prior to posting it!