ReadWriteWeb has extracted the most interesting detail from the release notes, the addition of support for local file handling APIs from the HTML 5 specification.
Another example that Mozilla uses in its documentation is photo thumbnails. Normally, you would have to upload the image to the site’s server before you could see a thumbnail. Now, developers can easily render and display these thumbnail before the file is uploaded.
This is a big feature for web site and application developers but also a concern in terms of security. I haven’t reviewed the HTML5 specification in any detail as of yet so do not know what constraints and limitations are in place to prevent exploits like directory traversals and other collection or alteration of local file data. I have to assume this was not undertaken naively. Even if the spec includes considerable security protections, I anticipate this is going to introduce a lot of security flaws before it stabilizes just because it will present such an attractive target.
RWW also notes another initiative coinciding with the beta 4 release, that of an add-on to help test and report on the compatibility of other add-ons. I installed it before upgrade to the 3.6 beta 4 release and I have to say that it definitely eases the biggest burden I’ve personally encountered in adopting new versions. It adds the ability to suppress the usual compatibility checking in Firefox’s add-on manager which is necessary for reporting on broken extensions in more detail than they simply aren’t compatible.
It has the additional side benefit of allowing incompatible but fully functional extensions to continue to run so the add-on upgrade experience becomes a lot smoother. In the past, I usually waited until well after a new version of Firefox was out just to make sure I didn’t lose any of my critical extensions.