Just checked on my Firefox on Android in settings/Homepage and when I unchecked everything there the tabs did not contain any links.
Though if advertising is the primary concern, then you only need to unselect "sponsored shortcuts" and "sponsored stories". Personally I find the others suggestions (of course not sponsored) useful, as those are pages I recently visited or bookmarked. There might be some pinned shortcuts, you can hold click and remove them.
Not possible to do officially according to Mozilla. I haven't found a setting yet on nightly to completely disable suggest, only to toggle the US specific features
This probably doesn't help, but on the off-chance it does... I don't have a smart device, so decided to check desktop Firefox.
Desktop FF definitely has the ability to turn off showing open tabs in the suggestions; it's in the Privacy & Security settings under the Address Bar heading.
Curiously, the attribute name in about:config is called browser.urlbar.suggest.openpage, which doesn't mention tabs specifically, but it's the right setting.
This makes me wonder if there's the same terminology difference on Android leading to you thinking it's not the right option, or if there's an about:config entry there which does something similar, even if there's no direct way of getting at it.