Tabs Reopening Later On Their Own

Since the last update I've noticed that tabs I have previously closed will be open again later. I'll close them and they still come back sometimes up to 3 or 4 tabs at a time. In order to get them to stay closed I have to go to open a new tab, tap the 3 dot menu (help I'm blanking on the jargon for it -- it isn't lines so not a hamburger) and press the tab's title in the list under highlights and either Remove or Delete from History. I don't have any add-ons installed besides one automatically installed by Mozilla. I'm on a Google Nexus 5X in case that affects versioning, etc.