A year and a half after the last major Firefox release, Firefox 3 Release Candidate 1 is here with  a very long list of new features and improvements.

For those who have been following Firefox 3 development you may want to jump to What’s new in Release Candidate 1.

For the whole scoop, make yourself comfortable and read on.

Performance

Firefox 3 gets several performance improvement gains. Among them, profile guided optimizations (PGO) provides an optimized Firefox build based on the way it internally works. So far it is only available for Windows. Linux should follow shortly and Mac OS X could also make it before final release.

A very noticeable gain can be seen in JavaScript: it is about three times faster than Firefox 2 implementation. JavaScript is twice as important for Firefox since it is not only used in a large number of web pages but it’s also what makes Firefox UI work.

Memory wise, a much needed memory cycle collector is now in place to take care of freeing memory no longer used by modules that requested it but failed to release it properly. This and other memory oriented tweaks, seem to have paid off so far: a set of tests I recently ran suggests a noticeable increase in memory management efficiency with more memory freed as tabs and windows are closed and no mysterious memory eat up when Firefox is idle as it has been reported several times in the past for Firefox 2.

Places

A much announced and expected feature is Places, the integrated history and bookmarks manager interface powered by SQLite, a small open source database engine that provides much more robust querying capabilities.

With Places you are able to search your history, tags and bookmarks with a fully fledged search interface. You can select where to search (History, Bookmarks Menu, Bookmarks Toolbar, All Bookmarks or the selected folder), and then save the search as a Smart Bookmark that updates as your personal web grows and changes.

The Library, Places manager, also adds backup and restore UI so it is easier to recover a damaged file or incorrectly deleted bookmark.

Tagging is a new Firefox feature tightly related with Places and some of the changes to the location bar: click on an empty star icon in the location bar to save the current page as a bookmark. Click it again and you can specify a folder to save the bookmark to, create a new one and add tags you can later search on.

The Location Bar

The autocomplete list that appears when you start entering letters in the location bar is no longer restricted to web addresses but also looks into bookmark and history page titles and tags which make it more comprehensive.

Suggestions are shown in two different lines and colors for page titles and addresses, which according to studies on human cognition, makes it easier for us to focus on what we are looking for. So, if the user knows she is entering part of a web address or a page title it will be easier for her to find what she’s looking for. Highlighting the match result also helps to direct the user’s attention.

It features adaptive learning so after a few repetitions Firefox will learn what letter combinations you use for what site and will provide better suggestions. This should address the case where frecency (a combined frequency and recency index) didn’t provide the best results.

You can also perform multiword searches, so for example “firefox downloads” filters address, titles and tags containing both words but not necessarily in that order.

I believe this feature alone is the best Firefox 3 has to offer, justifies Places large resources investment and will become a landmark in Firefox and web browsers in general development. Once you get used to it, there is no turning back.

Visual Refresh

One of the most visible changes are the theme updates in all platforms with a strong emphasis in making Firefox feel as a native application on each operating system.

On Windows XP and Vista, the theme is called Strata.

The Options window on Windows Vista with the new icons.

Mac OS X users get Firelight, a new Safari-like theme introduced with Beta 2, formerly known as Proto. Linux users get Tango, a theme that blends with Gnome native icons.

The back and forward buttons have been combined in a single keyhole-shaped widget featuring a single history menu and is now featured in all platforms except Linux.

According to the new guidelines, consistency across platforms is obtained through icons shape while OS integration is provided by texture. In Linux case, it’s very hard to set one due to the many available distributions and their particular themes.

There’s an invisible splitter between the location and search bars that you can use to set their width when they are placed next to each other.

Developers are aiming to deliver better operating system integration in Firefox 3. This will be most notable for Mac OS X and Linux users who will now get native widgets like text boxes, menus, check boxes, icons, button order and orientation following each OS guidelines.

Mac OS X users get integration with Growl, a popular centralized notification system, while Windows Vista gets native looking menus and blue icons that blend better with overall Vista look.

See more Windows XP, Vista and Linux screenshots in this recent post.

The Page Information dialog has been reviewed to become more organized and informative and allows to set all site specific preference from a single location.

Tabbed Browsing

On tabbed browsing, Firefox will not only warn you when closing several tabs and windows at once but will prompt if you want to save the currently open tab set: a good catch and a good way to introduce this helpful feature to new users.

Tabs now scroll smoothly.

You can duplicate and move tabs (including its history and current status). To clone, press Ctrl while dragging a tab, or just drag them across windows to move. A very helpful addition!