Mozilla late last week began offering Firefox 3.5 to users still running older versions of the browser, the first of what will likely be several attempts to get people to upgrade before support expires for Firefox 3.0.

Firefox 3.5.2, the current version of the edition launched last June, is now being offered to users running 3.0.13, the most up-to-date edition of Mozilla’s 2008 browser.

Although users have been able to upgrade to Firefox 3.5 for two months by downloading the new browser or manually triggering “Check for Updates” under the Help menu, Thursday’s offer was the first automated update pitch.

When Firefox users get the update offer, they can choose between accepting the update, postponing it 24 hours or declining it. The latter, however, doesn’t permanently block future offers. “We might send you another offer again in the future, but it won’t be for several weeks or months,” explained Mozilla on its developer center.

Last year, Mozilla launched what it calls a “Major Update” offer several times.

Not all users of Firefox 3.0.13 will see the upgrade offer at the same time; Mozilla rolls out such offers slowly at first to make sure its download network can take the traffic.

Firefox 3.0 users have until January 2010 to switch to the newer Firefox 3.5. At that time, Mozilla will halt production of security patches for the older version.

Last year, Mozilla offered the major update for Firefox 3.0 about four weeks after it launched the browser. The delay this time around — the offer for Firefox 3.5 follows that edition’s debut by about six weeks — was not explained by Mozilla.

Mozilla’s eagerness to get users onto the newest Firefox is in stark contrast to Microsoft’s policy with Internet Explorer (IE). Even though grassroots campaigns that urge users to dump IE6 are gaining momentum, Microsoft said last week it won’t abandon the eight-year-old browser. Several managers in the IE group have cited enterprise reliance on IE6 as the reason why it’s not feasible for it to push people to a newer model, such as this year’s IE8.

According to data from Web measurement company Net Applications, about 20% of all Firefox users are already running 3.5, while slightly more than 72% are still using Firefox 3.0. The remaining 8% are running versions older than Firefox 3.0.

Hot on the heels of the release of Safari 4 comes the final version of Mozilla’s Firefox 3.5. Originally slated to be a “.1″ increment to Firefox 3, the number of improvements slated for this release earned it a bump to a full “.5″ upgrade. When compared to the changes from Safari 3 to Safari 4, however, it seems Mozilla could just have easily named this one Firefox 4, as there are a multitude of new and improved features.

Surf faster

One of the more-touted new features is speed–it seems we browser users can never have enough. Running theSunSpider JavaScript benchmark, Mozilla claims that Firefox 3.5 is more than twice as fast (at that particular benchmark) than was Firefox 3. There have also been improvements made to the rendering engine, designed to help display Web pages more quickly than did Firefox 3. Finally, Firefox 3.5 also requires less real memory to run, and manages that memory better while in use, so it should load more quickly and not bog down as much over time.

I ran my own simple SunSpider benchmarks, not optimizing or controlling anything, and my results (on my 2.66GHz Mac Pro) back Mozilla’s claims. My results were more than two times faster in Firefox 3.5 than in Firefox 3–1103.6 for Firefox 3.5 versus 2742.6 in Firefox 3. (In these tests, lower is faster.) Safari 4, though, is still the SunSpider speed champ, clocking in at 590.2 on my Mac Pro.

You’ll see more benefit from these improvements on newer hardware–running the same test on my ancient-but-beloved PowerBook G4 (1.25GHz, 768MB RAM) showed only about a 10-percent difference between the two Firefox releases, and Safari 4 was then only about 10 percent faster than Firefox 3.5.

I haven’t used Firefox 3.5 long enough yet to comment on the memory usage and rendering improvements, but there’s no doubt it’s a snappy browser. Then again, I never had any real complaints about the prior version of Firefox. More time is needed with the browser to see how much improvement has been made in these areas.

Surf securely

For those who use public computers, or otherwise wish to keep their browsing habits confidential, Firefox 3.5 includes a new private browsing mode. When enabled (via the Tools -> Start Private Browsing menu item). One nice touch here is that when you enter private browsing mode, Firefox will store all of your open windows and tabs, then make them vanish while presenting a new blank browser window. When you exit private browsing mode, Firefox will restore your saved windows and tabs. When in private browsing mode, Firefox won’t retain a record of visited pages, form or search bar entries, passwords, downloaded files, cookies, or web cache files. You can still create bookmarks, and these will be retained when exiting private browsing mode.

There are other privacy-related enhancements, including the ability (when viewing history) to “forget” a site, which will erase all record of your visit to that site from the browser. Instead of clearing all your history, you can choose to clear recent history, which lets you pick the time range to clear, including the last hour, two hours, four hours, all of the current day, or everything. You can also specify which bits of history you’d like to clear.

Surf by location

One of the more interesting features in Firefox 3.5 islocation-aware browsing. This optional feature lets Firefox provide your location to location-aware web sites, such as Flickr’s map. When run in Firefox 3.5, the key area of the map as a button labeled Location. Click it, and Firefox will ask you if it’s OK to share your location with Flickr. If you approve, Firefox will try to determine your location.

To find out where you are, Firefox uses both your IP address and the location of nearby wireless access points to determine your location. (The iPhone and iPod touch also use wireless access points as one method of determining location.) I was only able to test from home, and it worked quite well from here (on Flickr’s map page). As more Web sites become location-aware, I believe this feature will become even more useful. (Opera also offers location-aware browsing.)

Other changes

Behind the scenes, there are myriad changes designed to improve your browsing experience. In addition to the already-noted changes to the page rendering engine, Firefox 3.5 supports downloadable fonts, and HTML 5’s local storage, offline applications, and the new audio and video elements that make it much simpler to include audio and video on a Web page. There’s much more for developers, if you’re interested in the nitty-gritty details.

There are, of course, numerous small improvements. The awesome bar–Mozilla’s name for the browser’s location bar–is now awesomer (more awesome?). Tab handling has been improved, and now features Safari-like abilities to drag-and-drop tabs on and off the tab bar, and to rearrange them by dragging. Session restore can now recover even the text in a web form you were typing in when your machine or Firefox unexpectedly quit. Firefox is now available in more than 70 languages, and there are another dozen or so in beta.