This is [patched](https://blog.mozilla.org/security/2010/03/31/plugging-the-css-history-leak/) by Firefox. Although there are [limits](https://dbaron.org/mozilla/visited-privacy#limits) to how well Firefox can protect you. You can get extra protection by setting `layout.css.visited_links_enabled option` in `about:config` to `false`, which will disable styling of visited links.