With 8.x mobile software I was able to configure LEFT button in IE as shown in the screenshot (star/favorite button).
How do I configure this option in wm10?
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This option cannot be configured in Windows 10 mobile i.e) in Edge. This button will be there for tabs only.
Possibly not unless there is a heavy demand.
Here is the reason. Edge browser shares a large amount of source code between PC and mobile. Left button is currently the only way to access the Tabs. The other options possible for the left buttons such as Favorites, Reading list, History and Downloads are integrated into a single Hub. This hub is shared between PC and Phone. The only thing left out of this Hub is the Tabs and there is no need for it in PC. Unless Microsoft implements a way to access tabs through an another way the left button will remain as it is i.e.) for tabs.
But who knows, Tomorrow is never written. If there is enough people asking for it in the Feedback Hub, Microsoft may implement it.