On a regular basis, we like to highlight the latest features in Firefox for developers, as part of our Bleeding Edge series, and most examples only work in Firefox Nightly (and could be subject to change).

Per Window Private Browsing

Private browsing is very useful for web developers. A new private session doesn’t include existing persistent data (cookies and HTML5 storage). It’s convenient if we want to test a website that stores data (login and persistent informations) without cleaning every time these cached data, or if we want to login to a service with 2 different users.

Until now, when entering Private Browsing in Firefox, it was closing the existing session to start a new one. Now, Firefox will keep the current session and open a new private window. You can test it in Firefox Nightly (to be Firefox 20). There’s still some frontend work to do, but the feature works.

Canvas’ globalCompositeOperation new values

The ‘globalCompositeOperation’ canvas property lets you define how you want canvas to draw images over an existing image. By default, when canvas draws an image over existing pixels, the new image is just replacing the pixels. But there are other ways to mix pixels. For example, if you set ctx.globalCompositeOperation = "lighter", pixel color values are added, and it creates a different visual effect.

There are several effects available, and more on them can be found in globalCompositeOperation on MDN.

Rik Cabanier from Adobe has extended the Canvas specification to include more effects. He has also implemented these new effects in Firefox Nightly. These new effects are called “blend modes”. These are more advanced ways of mixing colors.

Please take a look at the list of these new blending modes.

And here’s an example of how to use them:

JS Bin demo.

If you don’t use Firefox Nightly, here is a (long) screenshot: