Mozilla has released Firefox 4.0 RC 2 in March 18. This RC doesn't bring major updates, only localizations updates and one addition of localization. RC 2 also blacklist several invalid HTTPS certificates according to the Release Notes. The final version is expected to be released on March 23, so only few days left.
Since i'm running Linux, all of the preferences are stored in .mozilla, so i only need to copy that directory to other directory, for example .mozilla.old and start the firefox binary from Mozilla. There are three extensions that aren't compatible (yet) with Firefox 4.0, but it will be updated as soon as Firefox 4.0 final version is released in March 23.
The UI has been revamped so that it now has smaller area in above tabs, so more space on the view-screen. The menus are also being simplified.
This post is written using Firefox 4.0 RC 2.