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@banconi
Alternative solution:
[Applies only to those who use Cmd + T to open new tabs]
Instead of trying to replace new tab with other Extensions,
just set the function of shortcut "Cmd + T" to enable Momentum extension (go to opera://settings/keyboardShortcuts), replacing the original function of opening the default speed dial.
Simple. 
@banconi
Alternative solution:
[Applies only to those who use Cmd + T to open new tabs]
Instead of trying to replace new tab with other Extensions,
just set the function of shortcut "Cmd + T" to enable Momentum extension (go to opera://settings/keyboardShortcuts), replacing the original function of opening the default speed dial.
Simple.