By default, cmd + number in VS Code splits the window.

I’m more used to what Firefox does which is switching tabs.

This can be achieved with the following into keybindings.json

{ "key": "cmd+1","command": "workbench.action.openEditorAtIndex1" },
{ "key": "cmd+2","command": "workbench.action.openEditorAtIndex2" },
{ "key": "cmd+3","command": "workbench.action.openEditorAtIndex3" },
{ "key": "cmd+4","command": "workbench.action.openEditorAtIndex4" },
{ "key": "cmd+5","command": "workbench.action.openEditorAtIndex5" },
{ "key": "cmd+6","command": "workbench.action.openEditorAtIndex6" },
{ "key": "cmd+7","command": "workbench.action.openEditorAtIndex7" },
{ "key": "cmd+8","command": "workbench.action.openEditorAtIndex8" },
{ "key": "cmd+9","command": "workbench.action.openEditorAtIndex9" }

To open keybindings.json, open the command palette and select

Preferences: Open Keyboard Shortcuts (JSON)