ZDNet Australia revealed in December last year that tabbed browsing -- one of the most popular features of the Firefox, Opera and Internet Explorer 7 Web browsers -- had been developed for Thunderbird by software developer Myk Melez.
But work on that feature has now stalled.
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"Recently, there has been some development work in bringing Firefox's tabbed interface over to Thunderbird, hopefully by version 2.0," wrote developer Gary Kwong on Mozilla's Thunderbird development blog yesterday.
"However, due to circumstances, the volunteer at the heart of this new feature is unable to carry on as of the end of January," he continued.
"I'm being assigned away from Thunderbird work at the end of January, which unfortunately doesn't give me enough time to do the next phase of this work," said Melez himself in an online bug report for the tabbed browsing feature.
Kwong said if no developer was prepared to work on the feature tabbed browsing might not make it into Thunderbird 2.0.
"For those of you expecting this new feature, you may have to wait a little longer while this gets resolved," he wrote.
But Mozilla is still keen on tabbed browsing.
"The advantages of using tabs over windows is applicable to both applications [Firefox and Thunderbird] as using tabs have been shown to reduce memory usage, taskbar clutter and increased accessibility etc," wrote Kwong.
Thunderbird 2.0 is expected to be released around October this year, according to Mozilla.
The next version of the open source e-mail client is to provisionally include new features such as an improved ability to fight junk mail, custom folder pane views like favourites, unread and recently used, and the ability to 'tag' messages with labels.
According to Mozilla, Thunderbird has been downloaded more than 21 million times since its initial 1.0 release. The e-mail client can be downloaded from Mozilla's Web site and distributed freely. It is is available for Windows, Linux and Mac OS X.