This is important because the biggest obstacle to any new mobile operating system becoming a serious player is that the incumbents already have hundreds of thousands of apps and tens of thousands of programmers who know the languages they're written in. Writing "apps" for Mozilla OS-based phones, however, should be easy for anyone who already writes pages for the web. Thus, even if Mozilla OS has a tiny market share now, the phone could potentially gain a sizable library of apps.
In addition, because these are web apps, users of Mozilla OS won't be locked into the Mozilla platform--they could just as easily access their apps through the web browser of any other smartphone. That cuts two ways: It would make it easier for users of Mozilla OS in future to abandon it for another kind of phone, but now, when the OS has essentially zero users, the idea that there's no risk in trying it out could appeal to some people.
Copying Google's Chrome OS strategy
Mozilla's approach is, not incidentally, a mobile-phone version of exactly the strategy behind Google's Chrome OS. Chrome OS competes with Windows and Mac OS X, but runs only web-based apps. That way it doesn't need to rely on software companies building new word-processing, spreadsheet, photo-processing and all the myriad other kinds of software people use on their computers; web versions of all these things exist already.
So why is Google building a web-based desktop OS but not a web-based mobile OS? For one, a web-based mobile OS is still quite experimental, and it's not clear Google could have competed with Apple as well as it has if it had opted for one in the first place. Second, a web-based mobile OS is a pretty radical idea, and it could simply be the case that Google just didn't have enough foresight to pursue one back when it was developing the (then still radical) concept of an open-source mobile OS, Android.
But there's one more reason Google might not be pursuing a web-based mobile OS of its own: It doesn't need to, because Mozilla OS already helps Google in two ways. First, Google is already the default search engine on Mozilla's Firefox web browser; in fact, as of 2010, the most recent year for which numbers are available, Mozillaearned 84% of its revenue from this arrangement, which in 2011 was extended for at least three more years.
Second, Google also makes a lot of the web apps Mozilla OS users are likely to use, such as Google Drive, its cloud storage, or Google Docs, its office software suite. And Google doesn't make money on software or devices, anyway; its driving business logic has always been to get people to use Google products as much as possible, wherever they are available--whether via an app, the web, or a web app. As far as Google is concerned, Mozilla OS is as close to being a mobile version of Chrome OS as makes no difference.