It seems to be a gzip related issue.
Server returns gzipped content from the cache but does not return “Content-Encoding: gzip” header;
Browser receives gzipped content but does not know that the content is gzipped.
Possibly some of rewrite rules or some directives in the W3TC created (modified) .htaccess file is not supported by your server. Possibly ForceType directive in htaccess file is not accepted by your server.
You should try to find out all directives in .htaccess are working. Look at server error logs or contact with your server administrator.
You can turn off gzip from w3tc browser cache options but it causes a significant performance decrease.
https://superuser.com/questions/443952/firefox-shows-chinese-strange-encoding-characters-on-pages
