I often find myself switching between two tabs while researching something or gathering info from one tab to use in another.
It would be useful to be able to double tap the tab icon in Firefox mobile to switch to last used tab, instead of having to open the tab chooser and then find the previously used tab.
On Samsung mobile phones, and possibly other Android phones as well, you can double tap the "recent apps" button to switch to the last used app. I find this very convenient and intuitive and would like to see same function for tabs within Firefox.