Problem with MIME type: css

Hello.

In first, this is my Apache configuration:

Server Version: Apache-AdvancedExtranetServer/2.0.50
(Mandrakelinux/7.2.101mdk) mod_perl/1.99_16 Perl/v5.8.5
mod_ssl/2.0.50 OpenSSL/0.9.7d PHP/4.3.8


I have problems with .css files
The same problem is reported here:
http://lists.evolt.org/archive/Week-...22/166630.html

To resume:
I have a web page whith a line:
<link rel="stylesheet" type="text/css" href="myfile.css">

It works fine! but...
With Firefox 1.0.1, if I recall this page 2 times (for example), I have
an error:
Firefox says that the file 'myfile.css' is text/plain and not 'text/css'!

So, I sniffed the network and the result is:

The 2 first time, Apache answers for "myfile.css":
HTTP/1.1 200 OK (text/css)
And the 3rd time:
HTTP/1.1 200 OK (text/plain)

Do you know this problem?
Have answers?

Thanks by advance
Fab
:-)