The news comes as some embarrassment for Mozilla, coming just a few days after it boasted Firefox 3.5 was the 'most popular browser' though some creative market share analysis.
While the 3.6 and 3.7 versions of the browser will feature security and stability enhancements - not least the long-awaited ability to run browser tabs in separate processes to prevent the whole thing falling in a heap each time one tab crashes - Firefox 4.0 will feature a Windows 7 look and feel, albeit almost a year after the Microsoft product hit the market.
Industry watchers say the delays will no doubt be welcomed at Redmond, as Internet Explorer 8, whose market share is growing steadily alongside the uptake of Windows 7, will now have a clear advantage over the increasingly clunky-looking Firefox and the bit-part players Safari and Chrome.
Mozilla will no doubt also be kicking off an internal inquiry as to why the point release of Firefox 3.6 has so far warranted no fewer than five public betas.