Mozilla is sticking to its new fast-track development cycle, with plans to release a beta version of Firefox 5 on Tuesday, May 17.
Meanwhile, the company is also working on phasing out Firefox 3.5.
Firefox 5 is currently in Mozilla's Aurora channel, but will move to beta tomorrow. Aurora is a recently created channel that now comes between nightly builds and beta in order to "deliver features to users at various levels of quality and polish," Mozilla said last month.
Firefox 5 for Mobile is also scheduled for a beta release tomorrow, according to the MozillaWiki.
Firefox 4 made its debut on March 22, about a year after the release of Firefox 3.5. Perhaps taking a page from Google Chrome, however, Mozilla is now shifting to an 18-week release cycle, which means we'll see several updated versions of Firefox this year.
Firefox 5 is scheduled for a final release on June 21. Firefox 6 is currently in the nightly channel and is scheduled to move to Aurora on May 24.
On Firefox 3.5, meanwhile, Mozilla will automatically upgrade 3.5 users to 3.6.
"We don't think self-selection will ultimately get us to the place we need to be," Christian Legnitto, Firefox release manager, wrote in a note posted to the developer mailing list. "We will force 3.6 on 3.5 stragglers not choosing to update to Firefox 4 or 3.6."
"We feel comfortable making the major update choice for users because a) the versions are very similar and b) we'd rather lose a small amount of miffed users than leave a large amount of users vulnerable," he continued.
On the Firefox 3.5 end-of-life Web site, Mozilla said it can't support the browser "into perpetuity." The company said it was frustrated with efforts to get users to move away from 3.5, and is "worried too many people do not upgrade and are on vulnerable and unsupported versions of Firefox."
At this point, 12 million people are on Firefox 3.5. But Mozilla said the upgrade from 3.5 to 3.6 is the "perfect storm" for automatic updating since it did not drop support for any platforms between the two versions, there is 95 percent add-on compatibility, and there are virtually no UI changes.
Legnitto did not discuss when this forced upgrade might occur; "No real actions need to take place until June," he said. The launch of Firefox 3.6.18, however, is scheduled for June 21.