Mozilla is working on a new exciting feature coming soon to Firefox on desktop and mobile platforms, which would serve compressed and optimized data to the users through a proxy server similar to Opera Mini, Google Chrome and Amazon’s Silk browser. Hence, users would enjoy faster web browsing with reduced bandwidth consumption. This feature is currently being developed under the project name “Janus” and available as a standalone add-on for Firefox. It may become an integrated feature of Firefox in future builds.
Once Janus is enabled by the user, Firefox routes all network connections through a proxy server which serves compressed and optimized data. This reduces data consumption and load time, increases user privacy and responsiveness of websites, improves overall browsing experience. Users can opt to block advertisements and enable GIF to WebM video conversion (which reduces file size). The proxy server compresses text and images, reduces HTTPS round trips to reduce latency, would use caching and SPDY protocol.
Janus is currently available as experimental add-on, and works only with the nightly build on desktop and Aurora build on Android. Install it from AMO, and you would notice its icon on the toolbar (on desktop) or addressbar (on Android). Users can see bandwidth stats and set other options from the Add-on Manager page.
Chrome on Android and iOS also offers similar data compression feature, but it doesn’t interfere with the secured/SSL-enabled sites. It can be turned-on by visiting Settings -> Bandwidth Management -> Reduce data usage.
Refer this wiki page for more information on project Janus.
Update: Mozilla has discontinued this experimental project.