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You can delete individual bookmarks by long pressing on them in the bookmarks view. https://support.mozilla.org/en-US/kb/get-started-firefox-android#w_use-the-awesome-screen-to-quickly-find-what-you-need
If you are willing to reset Firefox completely (resetting your bookmarks, history, passwords and other data) this can be done by simply uninstalling Firefox and reinstalling Firefox from the Play Store.