As far as I can tell, the tab pickup/open tabs from other devices list on the about:firefoxview page only ever shows 3 tabs. Why? I want to see many more.
Agree! Just started using Firefox mobile over Safari, and this feature is what sold me. It would only make sense, because when you're viewing mobile, you can already see all the tabs you have open on the web.
There's also a little workaround to this, by clicking on the Firefox account icon in the web browser, it will show a list of all the tabs you have open on mobile.
I used Firefox across Mac, Android and Windows and use sync all the time but the tabs are hidden away in dropdowns - real pain if you want to open multiple tabs at once.