There was a bug closed for this problem,but the solution is like
# On some specific Linux installations you could face problems with Firefox.
# It could give you errors when loading the editor saying that some illegal
# characters were found (three strange chars in the beginning of the file).
# This could happen if you map the .js or .css files to PHP, for example.
#
# Those characters are the Byte Order Mask (BOM) of the Unicode encoded files.
# All FCKeditor files are Unicode encoded.
#
# Just rename this file to ".htaccess" and leave it in the editor directory.
# There are no security issues on doing it. It just sets the ".js" and ".css"
# files to their correct content types.
But how can we name the file into .htaccess in windows?
# On some specific Linux installations you could face problems with Firefox.
# It could give you errors when loading the editor saying that some illegal
# characters were found (three strange chars in the beginning of the file).
# This could happen if you map the .js or .css files to PHP, for example.
#
# Those characters are the Byte Order Mask (BOM) of the Unicode encoded files.
# All FCKeditor files are Unicode encoded.
#
# Just rename this file to ".htaccess" and leave it in the editor directory.
# There are no security issues on doing it. It just sets the ".js" and ".css"
# files to their correct content types.
But how can we name the file into .htaccess in windows?