Mozilla triggered an automatic update to Firefox 3.0 late Monday, a week after the company said it would start offering the newest version of its browser to users of the older Firefox 2.0.0.16.

Mike Beltzner, the director of Firefox, announced the planned update in a post to the Mozilla Developer Center blog. "Starting a little later tonight, users with the latest version of Firefox 2 will be getting an offer to update to Firefox 3," said Beltzner.

Last week, Beltzner said that Mozilla had some last-minute snags, including localization chores, to solve before it could release the update.

Users of Firefox 2.0.0.16 began receiving update notices to Firefox 3.0.1 last night. According to other information published on Mozilla's site, about 75 percent of users will be offered the update at the start; depending on the server load, Mozilla will adjust that percentage up or down. "If you're running Firefox 2.0.0.16 you will see the offer in the next couple of days," Beltzner said.

The notice allows users to accept the update, postpone it 24 hours or decline it. The latter, however, won't be the last word: Mozilla may repeat the offer several weeks later.

Users who decide that they would rather return to the older version after upgrading to 3.0.1 can download and reinstall 2.0.0.16, Beltzner added. Mozilla will support the older edition with security patch updates until mid-December.

Firefox 3.0 shipped in mid-June, and features additions such as malware protection and enhancements that include a revamped address bar. When the browser went live on June 17, Mozilla's servers went dark under the load.

According to the most recent data available from Web metrics vendor Net Applications, Firefox accounted for 19.2 percent of the browser market share at the end of July. At the time, Firefox 2.0 users outnumbered those running version 3.0 by better than 2-to-1.