This should work:
RewriteEngine On
RewriteCond %{THE_REQUEST} ^(GET|POST)\ /content/profile/profile\.php\?username=(.*)\ HTTP
RewriteRule ^ /%2? [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /content/profile/profile.php?username=$1 [L]