Mozilla has just released the third beta version of its latest Firefox open-source browser, showcasing some of the upcoming features expected in Firefox 3.5.

The majority of changes appear to be on the back end, but there a few new things to look out for from a user perspective, including a 'private browsing' mode and easier tab management, as well as the ability to use special characters when navigating the smart address bar.

There are also a number of changes behind the scenes for web developers and extension creators. Developers have been given a host of new tools to play with, including the ability to test a site under development with the new TraceMonkey JavaScript engine to see how much faster it runs.

Extension developers, meanwhile, will have a new feature allowing them to test existing add-ons to ensure compatibility with the latest version of Firefox.

Installing this beta release will not overwrite previous versions of the browser, so if you're running Firefox 2.0 or 3.0 you will have two versions installed. This is to ensure that, should the beta throw up some fatal flaw, users will still have access to a previously tested and known version.

For those early adopters also using the beta of Microsoft's Windows 7, the Enter key no longer appears to have any effect when typing in a web address, even when selecting an address from the drop down list. Similarly, Ctrl+Enter doesn't add a 'www.' and '.com' at the beginning and end of text typed into the address field.

The most significant addition to Firefox is probably the inclusion of the private browsing mode, which is turned on by selecting 'Start Private Browsing' from the Tools menu. Any already open tabs or windows will be closed when the session is initiated.

While in private browsing mode no history of visited pages will be remembered, nothing entered into a search bar or text entry field will be added to the auto-complete list and no new passwords will be saved. According to the release notes, passwords should not be automatically filled in when in private browsing mode, but they always were when we tested it.

Cookies or other temporary files will be used during the private browsing session, but will be deleted once the session is ended. Anything downloaded will be removed from the download list, but the files themselves will remain, as will any bookmarks created during the private browsing session.

You can end the private browsing session by closing all open Firefox windows, or selecting 'Stop Private Browsing' from the Tools menu. The latter will then close all existing windows and tabs, and reopen any sites that were closed when the private session was started.

Mozilla has also added a privacy option called 'Clear Recent History' which builds on the 'Clear Private Data' feature available in the current version of Firefox. This allows users to clear existing browsing, saved form and search history as well as cache, cookies, offline web site data, saved passwords and authenticated sessions.

The new version offers the same feature, but with the addition of a time factor allowing users to remove data from the past hour, two hours, four hours, that day or the entire history.