No WYSIWYG-Editor (FCKeditor) after Firefox-Update (3.6)

This issue is valid for all OpenCms versions < 7.5.1.

If you have the problem like other users wrote on the mailing list:

...In my 7.5 workplace I suddenly find that when I open a page for editing 
only the simple text editor comes up. Not the FCK-Editor...

or

...since my customer upgraded to Firefox 3.6, he can not access the rich editor anymore. 
We use OpenCms 7.0.4...

This seems to be a year 2010 bug. For more informatioen please refer to [[1]].