A month and a half after Mozilla issued a significant update to its open-source Web browser, the organization has begun encouraging users of Firefox 3 to install version 3.5.

Firefox 3.0.13 users will see an offer to download the latest iteration of the newer version, 3.5.2, according to a blog post. Firefox 3.5.2 is available for Windows, Mac, and Linux.

Users can go ahead with the update, postpone the reminder, or shut it off altogether, but don’t expect this latter option to permanently mute the reminder. Firefox 3.0.x will stop receiving stability and security patches in January, so further coaxing will be likely.

Mozilla touts better performance in version 3.5, along with various features to make the browser a better foundation for running Web applications. But getting people to upgrade can be a problem. One of the biggest obstacles for Firefox, aside from the universal hassle of upgrading, is that Firefox extensions often break with a new version.

Mozilla, though, said more than 90 percent of Firefox’s add-ons now work with Firefox 3.5. Meanwhile, Mozilla coders are at work on Firefox 3.6, code-named Namoroka.

Browser upgrades can be a tricky issue. Microsoft is trying to coax users off Internet Explorer 6, a product now 8 years old. Google’s newer Chrome browser, by contrast, automatically updates itself to the newest version with no user intervention, though IT administrators can throttle the behavior.