This post originally appeared on the Mozilla Hacks blog.

Firefox 3.6 was released on Jan 21st and has already been downloaded more than 35 million times! It features a faster JavaScript engine, faster DOM performance and a bunch of new HTML5 features. Highlights for web developers include support for the WOFF font format, new CSS features like gradients and multiple backgrounds, drag and drop, File API, device orientation, and more. Want to see the full list? Look at our Firefox 3.6 for Developers page on developer.mozilla.org.

We hope that you took the time to test Firefox 3.6 on your web sites before the release. If you didn’t, it’s not too late to let us know about issues. Here are a few problems that we’ve seen in the wild so far:

• We’ve seen a couple of issues with the FCKEditor component, used widely on the web. It apparently doesn’t handle dates after 2009 properly (this is the first major browser release in 2010, a new decade!) and it also has some problems with document.readyState.
• People who had the YSlow extension installed saw crashes with Firefox 3.6. YSlow has been updated to version 2.0.6 to fix the issue
• Some Facebook apps were broken by a change we made to comply with the upcoming HTML5 standard. We updated element.getElementsByTagNameNS and document.getElementsByTagNameNS to no longer case-fold when doing tag name lookups. (We strongly suggest that you only use lower-case for tag names for many reasons, including this.) For more information, please see this note from Henri Sivonen about what’s changed. Facebook has since fixed the issue in their code, but other sites may also be tripping over this.

We’re looking for feedback on any developer-facing regressions you’ve seen in 3.6 from 3.5. Please comment on this post if you have feedback.