I want to gzip my static files. so put this in .htaccess:

<IfModule mod_deflate.c>
    AddOutputFilterByType DEFLATE text/text text/html text/plain text/xml text/css application/x-javascript application/javascript
</IfModule>

and looked for mod_deflate in phpinfo() output Loaded Modules section, and I found it. But when I track server responses with Firebug, no gzipped file can be found:

HTTP/1.1 200 OK
Date: Sat, 08 Sep 2012 21:41:21 GMT
Last-Modified: Sat, 08 Sep 2012 21:26:04 GMT
Accept-Ranges: bytes
Cache-Control: max-age=604800
Expires: Sat, 15 Sep 2012 21:41:21 GMT
Vary: Accept-Encoding
Keep-Alive: timeout=3, max=50
Connection: Keep-Alive
Content-Type: text/css
Content-Length: 18206

What's the problem? I'm sure I have mod_deflate enabled (according to php apache_get_modules()).

UPDATE: the request headers:

GET /d/jquery-ui.css HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20100101 Firefox/15.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache