Cannot modify header information

I keep getting this error when I try to login using any other browser except Firefox(on firefox I can autheticate). In Firefox I’m getting this error when I try to publish enything or, modify settings, etc.

The problem appeared from nowhere cause the day before everything worked perfectly… Thanks

Cannot modify header information – headers already sent by (output started at ……../wp-includes/default-filters.php:204) in ……../wp-includes/pluggable.php on line 850