Use RadEditor v5.x - v7.x in a hidden element in Gecko-based browsers
In scenarios where RadEditor is hidden in an element (div, table, etc.) by using style="DISPLAY:none", it cannot be used by end-users who use Gecko-based browsers. The problem is due to the fact that the editor cannot get focus. This is a known issue and the problem is not related to RadEditor but to the Mozilla browser. SOLUTION
To solve the problem, Telerik exposed a method in the editor's API - SetVisible(). You should explicitly call this method as RadEditor has no way of knowing that its parent DIV is visible or hidden.