Specially hard to read in dark mode. This is because "Read more preview" is a link and it seems sometimes that div itself gets focused when rendering the preview. Note the blue border around the div, if you click anywhere on the preview text, the focus is removed from the div (and becomes more readable on dark mode). This is not specific to Firefox only, I can see it on Chrome too.

First reported in T363481

Dev note: initially I thought this was related to https://developer.mozilla.org/en-US/docs/Web/CSS/:visited but not 100% sure about that now, just something to keep in mind