Taken together, the two projects provide a powerful step toward legitimacy for a technology that has been tarnished somewhat by its association with copyright piracy, but which is increasingly being used for wholly legal purposes.
"BitTorrent has been on our radar for some while, and when we looked at the protocol, we found that was a very good way of downloading files," said Christen Krogh, Opera vice president of engineering. "We consider it a natural extension of other download mechanisms."
The move into browsers, even without Microsoft's Internet Explorer, could accelerate BitTorrent's role as a de facto technology for distributing large files online.
Indeed, it was the addition of support inside the browser that helped technologies such as Macromedia's Flash become standard Web tools, rather than one of dozens or hundreds of plug-ins competing for surfers' and developers' attention.
The open-source BitTorrent technology was created by independent developer Bram Cohen in 2002, and has quickly risen to the top of the file-sharing world. Unlike Kazaa, eDonkey or other file-swapping networks, it does not allow searching other people's hard drives in order to find and download files.
Instead, it is primarily a file-download technology such as the venerable FTP (File Transfer Protocol). Rather than drawing files solely from one source, it connects the computers of numerous people who want the same file, allowing a computer that has already downloaded pieces of the file to automatically begin distributing those pieces to others.
That "swarming" mechanism, as it is known by programmers, allows files to be downloaded much more quickly than if there were a single source for the file. Red Hat Software has used the technology to distribute large files such as the Linux operating system to its customers.
Opera's support for BitTorrent is still in the "technical preview" stages, and the company is not providing a hard date for when it will be added to its core software. The company said that it has no legal concerns about the new software, despite the recent U.S. file-swapping companies.
"We're the good guys," Krogh said. "We are in no way encouraging people to illegally download material."
In an e-mail, independent Mozilla developer Kevin Stock said that the "MozTorrent" plug-in for Firefox was still in the "planning stages," but that developers expect the project to move quickly toward release once they finish building a user interface.
Reaching most consumers will ultimately require support from Microsoft, whose Internet Explorer retains nearly 90 percent of the browser market share, according to Web measurement and marketing company WebSideStory. Firefox's early rapid growth has slowed somewhat, leaving it at about 7 percent, WebSideStory recently said, while Opera's market share remains tiny.
Microsoft is in the early stages of building its own BitTorrent-like file download technology called Avalanche, which was by the company's Cambridge, England, research team last month.