OTRS Ticket#2019090810005009 is a report that math characters are not displaying properly in Firefox 67.0.4 on Linux. The math text is too light to read.
Another screenshot in the OTRS ticket shows that the reporter is receiving the fallback SVG and is not directly rendering the MathML. In any case, their browser renders the Mozilla MathML Torture Test correctly. Opening the SVG in a new tab results in the correct colors.
The issue persisted with Firefox in safe mode (addons disabled). They mentioned that the issue appears when a dark desktop theme is selected and the browser uses system colors.
The reporter was not able to replicate the issue in Google Chrome 76.0.3806.132. I was not able to reproduce the issue in Firefox 69.0 on Linux.