The third alpha of Firefox 3 is available now and while it’s still mainly oriented to get testers around the lot of work that is going on Gecko, Firefox’s web page rendering engine, we can find much more interesting features here.

As announced, support for animated PNG is included, so it seems to be on track to make it for Firefox 3. I wasn’t able to find any one I could try. Not even the samples provided by Andrew Smith, the developer who implemented APNG support.

Update: Andrew, let me know that the APNG format is still evolving fast and the samples, which are updated accordingly only work in the most current nighty builds.

Initial support for offline web pages. Not sure how far it is right now, but at least specific web page elements such as JavaScript code snippets, style sheets and images can be marked to be cached (saved) to be available while working offline.

Web developers can now restrict access to a served cookie to the originating server only and not local scripts. Thi will help prevent cross site scripting (XSS) attacks launched by malicious web sites that try to steal private information from another site.

An option to receive warnings when a web page automatically redirects to another when using the HTTP-Equiv=Redirect directive has also been added. It is turned off by default. To enable it, select Options in the Tools menu. Change to the Advanced page and check Warn me when web sites try to redirect or reload the page. Once activated, you will receive a message in the information bar when an autorefresh is prevented with a button to allow it if necessary.

While it could be helpful to prevent some web sites to abuse serving too many ads on a single page hit, it can very easily become annoying if the notification is shown for web pages with other more legitimate reasons. Hopefully this will become a per site setting to avoid the hassle.

Some other important improvements to how web pages are rendered have been added as well for stronger standards support.

As usual, the faint of heart should stay away from this and any other alpha release as there’s no guarantee on what it will do. I guess there must be at least two other alphas to go as a lot of features are waiting to be implemented. Firefox 2 stayed in alpha until all features got in place and this should be the same pattern for Firefox 3.