They are four applications designed to serve different purposes: A web browser, a music player and organizer, another that does the same for video, and a word processor for screenwriters. Yet they share one thing in common: All were built with a Mozilla-based toolkit, either the Gecko Runtime Environment or its successor, XULRunner. Both toolkits use the same codebase which runs Firefox.

Like Firefox, these applications are free and their code are released under open source licenses. None is affiliated with the Mozilla Corporation. Each is backed by an organization that plans to make money through its software.

Among this group, Flock has gotten the most publicity. Essentially, it is an unofficial version of Firefox with features added to make interacting with social, media and blogging sites easier for the user. Copying the successful business model that has worked well for Mozilla through its Firefox web browser, Flock was designed to bring in referral revenue through its search box.

Celtx is a screenwriting tool and filmmaker's pre-production suite. Its developers are working on several money-making angles. They are partnering with a film school, starting an online marketplace to cater to the needs of filmmakers, and licensing their server code.

"We took the approach of waiting to see who was using our technology before deciding what revenue models to employ. Now that we've grown to where we are, the revenue models are forming on their own," says Greg Dawson, vice president of software production for Celtx.

Miro comes from a non-profit entity, the Participatory Culture Foundation, but its developers are also looking to make some cash. They recently launched a co-branding initiative as one way to financially sustain development of its video application, which helps the user find, download and watch video content on the internet.

Though there are other toolkits that the developers of these projects could have used, the very existence of the Mozilla libraries--and the positive reputation of Firefox itself, in fact--inspired many of them to create their application in the first place.

"We were inspired by Firefox. Firefox was a direct inspiration to Songbird," says Rob Lord, CEO of Pioneers of the Inevitable, the team behind Songbird. Their Mozilla-based application is a mash-up of a web browser and music player, wrapped in a user interface that bears more than a resemblance to Apple iTunes'.

He elaborates: "A lot of things of the whole Mozilla approach we really liked: We liked the open source, open standards, open web approach. We liked the philosophy. We liked, from a product perspective, this was a rich internet application platform that was being tested by 80-to-100 million active users of Firefox."

For the Flock team, building their "social web browser" with the Gecko Runtime Environment was a pragmatic decision. Trying to code a brand-new browser from the ground up -- one that needed to be stable, standards-compliant and cross-platform -- would have taken several years.