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Inspecting the configuration of the Custom menus will show that they use the command "explorer" .exe to invoke the windows shell, . This is not to be confused with Microsoft Internet Explorer. This results in . The explorer.exe executable refers to the Windows file explorer which, when invoked with a URL parameter such as those provided by the MettleCI menu items, will result in the current user's default browser being invoked by the menu itemsto load the supplied URL’s.
To change the browser used to access MettleCI Workbench then simply change your default browser and this will automatically be invoked.