This seems intentional.
The Mozilla article
privacy-related changes coming to CSS :visited
describes these changes whose purpose is to avoid a page detecting which links
on itself were already visited, probably meant as an anti-marketing measure.
This change is described as:
You will still be able to visually style visited links, but you’re severely limited in what you can use. We’re limiting the CSS properties that can be used to style visited links to
outline-color and the color parts of the
stroke properties. For any other parts of the style for visited links, the style for unvisited links is used instead. In addition, for the list of properties you can change above, you won’t be able to set rgba() or hsla() colors or transparent on them.
The article also claims that "The impact on web developers here should be minimal, and that’s part of our intent". However, in the list of possible problems is mentioned (the bold is my emphasis):
- If you’re using background images to style links and indicate if they are visited, that will no longer work.
- We won’t support CSS Transitions that related to visitedness. There isn’t that much CSS Transition content on the web, so this is unlikely
to affect very many people, but it’s still worth noting as another
vector we won’t support.
As this is a new feature, you might file a complaint on the Firefox Forums.
Realistically, the only options I can see are to either accept the default schema
or move to another browser.