You need to use custom userChrome theme, add this line:

.toolbarbutton-1 .toolbarbutton-icon{ filter: drop-shadow(0 0 1px black) }

This will fix issue

You need to use custom userChrome theme, add this line:

.toolbarbutton-1 .toolbarbutton-icon{ filter: drop-shadow(0 0 1px black) }

This will fix issue