Open newtab rather than firefoxview when unloading the last tab

In bugzilla bug 1946432 they added a behaviour where about:firefoxview or about:newtab is opened when the last tab is manually unloaded by the user.

We want the about:newtab path, but it still calls FirefoxViewHandler.openTab (which we short-circuit) because FirefoxViewHandler.button is truthy.

We can either change this logic, or we can finally remove the firefoxview button from the customised toolbar.