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.

Again, apologies for potentially useless answer.