Empty contenteditable fields not accessible with mouseclick in Firefox

I was about to abandon this post as I discovered it only concerns Firefox and not IE9 or Chrome. But finally it would still be appreciable if solved as I use Firefox because of the Firebug debugging tool. I store and retrieve data on a page from MySQL tables (inline edit). Some fields are per definition empty NULL. I cannot start editing these fields as the mouse click doesn’t open the editor. There is a workaround if another field is editable, by moving from that field forward with TAB or backwards with ALT+TAB. To have a better idea of the area that each field occupies, I have a hover class adding yellow background. That doesn't show either on empty fields. For some fields I could add a default text but I would prefer to avoid that. If a field has contained text and if I delete that text (checking with Source that nothing is left), it will remain editable….until I do a page reload (F5)…