Firefox Nightly is where the exciting things happen. Firefox 19 is currently under heavy development and is getting new features and changes all the time, until it's ready to move up to the Aurora channel.
Obviously, not all of the new features that land in Nightly make it to the Aurora channel, some may be scrapped entirely.
With that caveat out of the way, here are some of the most recent additions to the Firefox Nightly channel.
Gecko, the HTML rendering engine that powers Firefox, now supports new length units, useful when designing a modern website or app.
Gecko now recognizes vh, vw, vmin and vmax. They represent the viewport height, width and the minimum or maximum between the viewport height width and height. 1vh represents one percent of the viewport height.
The need for such units arises when trying to design an element that depends only on the size of the page, not of its parent elements.
Another recent addition is the ability to export the Canvas view as raw data, i.e. an image. Gecko already offered the possibility, but via a "proprietary" method.
Firefox now supports the standard .toBlob() method. Developers will be able to export screenshots of their games, for debugging or as a feature.
"Blobs can be used by different APIs, like the File API or IndexedDB. We can create an alias URL the refers to the blob with window.URL.createObjectURL, which can be used in place of data URLs in some cases (which is better memory wise)," Mozilla explained
Finally, support for the PeerConnection API, part of WebRTC, is built into Firefox 19 Nightly, as well as into Firefox 18 Aurora. It's not enabled by default in either of them, you can find out how to enable the feature and test it here