Do you mean you want to close some of the open tabs, or you want to remove the history of previously open tabs? To close tabs, tap the number in the address bar and then you can use the little X's on the tab thumbnails to close them. More info: [[Using Tabs on Firefox for Android]]. To clear history, you can go into Privacy settings as described in this article: [[Clear your browsing history and other personal data on Firefox for Android]]. I think it would be covered by "Browsing history".