For Firefox on Android: Use menu > Settings > Privacy > Clear Private Data feature as described in this article: [[Clear your browsing history and other personal data on Firefox for Android]]. Be careful to uncheck categories of data you don't want to lose. If the history is not removed, do you recall every modifying any of Android's hidden developer settings? We had a thread where that was the problem, but I don't have the link handy. For Firefox on Windows: Try the "3-bar" menu button > History > Clear Recent History as described in this article: [[Remove recent browsing, search and download history]]. Be careful to uncheck categories of data you don't want to lose. Does that work?