The issue is that on some systems Firefox displays the checkbox and radio button controls without an outline.
These controls can still be selected and functional, but they are difficult to see and sometimes this can cause problems with completing forms.
I have run into this issue a number of times, especially on Linux-based systems, and this occurs when the Firefox Profile is using the default system provided styling.
The following steps fixes this issue on Linux by providing a custom styling.
1. Navigate to the Firefox profile folder location.