With RadEditor for AJAX Version 2013 Q3 it is not possible to edit tables in Design view. Clicking within a table seems to cause the editor to lose keyboard focus, the cursor is not visible. The problem is reproducible on the Telerik WYSIWIG Editor demo page.
You have tested every version of FF since 10.0.3? I just tried 13, 15, 16, and 17ESR. I see the problem up through (and including) 16. 17ESR does not exhibit the problem. The page you reference is the Firefox current ESR version page. I could not find what they officially support. I believe their position is 'we only support the latest version'. We cannot use the DIV work around. We incorporate the RadEditor for creating/editing HTML email and DIVs are not supported/rendered by Email clients. While I agree that the user should upgrade to a more recent browser for security reasons, I have no control over our customers IT environments. Being new to Telerik, I'm curious about something. By declining the suggestion, a known problem will not be publicly visible? The first thing I do when investigating a problem is check the support database. If info available there is based on what you accept and decline then the value of the data is degraded.
The issue is reproducible, although only under this version of Firefox, which accordingly to the following list, currently is not supported from Mozilla: http://www.mozilla.org/en-US/firefox/organizations/all.html Possible resolutions are to set the editor's content area mode to Div (you can test it with this online demo: http://demos.telerik.com/aspnet-ajax/editor/examples/contentareamodediv/defaultcs.aspx) or upgrade the used Firefox version.