In a SharePoint 2010 list, the Data Sheet view option is grayed-out or a view set to use Data Sheet displays a message that ActiveX controls are not supported.
SharePoint 2010, Firefox, Chrome, any non-IE browser
In SharePoint 2010, the Data Sheet view uses ActiveX controls which are an old Microsoft technology not supported in modern browsers.
To view a Data Sheet list, Internet Explorer must be used.
SharePoint 2013 uses a different technology and Data Sheet is now Quick Edit and can be used in any browser.