Firefox 18 also implements basic support for WebRTC, which will eventually allow compatible browsers to perform certain high-end functions such as video and voice calls without the need for third-party plugins.
What’s New in Firefox 18?
Support for Retina Display on OS X 10.7 and up.
Preliminary support for WebRTC.
Experience better image quality with our new HTML scaling algorithm.
Performance improvements around tab switching.
Improvement in startup time through smart handling of signed extension certificates.
Disabled insecure content loading on HTTPS pages.
Improved responsiveness for users on proxies.
Apart from these features, there are some bug fixes and other smaller features. You can read full release notes from here Official Firefox 18 release note.
You can download Firefox 18 offline installer from the link below.