Mozilla still expects to release Firefox 3.5 Beta 4 sometime later this week, despite not having specified an exact date.

Last month the software company revealed it was renaming version 3.1 of its Firefox web browser and planned to call it Firefox 3.5 instead.

Mozilla said the name change was prompted in part by developers who thought that the '3.1' moniker didn't properly reflect the amount of new features and changes from last June's Firefox 3.0.

Beta 4 looks set to be the last test version before Mozilla moves on to the release candidate of Firefox 3.5. However, Mozilla has not made good on many Firefox deadlines. Earlier this year, Mozilla delayed the release of Beta 3, not once but twice, before moving on to Beta 4.

Most of the previous delays stemmed from Firefox's new JavaScript engine, TraceMonkey, which promises to speed up page loads two times faster than Firefox 3.0 and nine times faster than Firefox 2.0.

At the moment, Beta 4 contains seven high-priority bugs requiring fixes, five of which involve the JavaScript engine. When it is finally ready for mass adoption, Firefox 3.5 promises some new features largely based on HTML 5 - the latest update to the core language of the web.

Mozilla says Firefox 3.5 will deliver noticeably improved performance, security and a better user experience overall.

While Mozilla works to resolve problems with Firefox 3.5, the foundation is also hard at work on Firefox 3.0.Just this week, Mozilla released Firefox 3.0.9 update with 12 security fixes - four of which were critical.