By default on Firefox 4 the home icon has moved to the right hand side of the navigation toolbar.

If it is not there follow the procedure in the following link to drag the home icon back onto the toolbars - https://support.mozilla.com/kb/How+to+customize+the+toolbar

Alternatively use the method to restore the default toolbar set, or reset toolbars and controls as shown here - https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing

home page icon has now disappeared

Use the method to reset toolbars and controls as shown in the last link I posted. It will restore the toolbars to their default settings, including having a home icon