You will, of course, be limited to the APIs available in standard Javascript. At the moment that means missing a lot of things that people might want to be able to do (or need to be able to do to write things that act like freestanding apps instead of websites). The way Mozilla tried to get around that was to add a lot of Firefox OS specific APIs and then claim that those APIs were part of 'the open web' because they were going to submit them for standardisation at some point in the future.
Mozilla seem to think that they are the sole arbiter of what the web standards are. The APIs they want to add are 'open' and 'standard' and anybody who is against adding those APIs is an entryist enemy of 'the open web'. APIs proposed by other vendors that Mozilla dislikes are an attack on the open web.
That's not how it works.
Things only become standards (formally or de facto) when all (or nearly all) of the big browser vendors support them. That's the definition of an open standards based platform - it's lowest common denominator. If you don't like that reality then you don't like open standards based platforms. There was little sign that the other browser vendors had any interest in adding a bunch of FirefoxOS specific APIs (just as how Mozilla have rejected various proposals from other vendors over the years).
You can argue that all this is very unfair. Why should Apple and Google and Microsoft have veto power over what APIs are available on FirefoxOS? But this is a trap of Mozilla's own choosing. It's the fundamental problem with the dream of a single platform standard for all user facing software. It creates a tight sandbox (both software architecturally and politically) that gives the big players veto power over everything. The idea of a single grand standard platform with perfect compatibility across all devices is certainly alluring, but it has huge downsides that nobody ever seems to really want to talk about. And that's before you get to the technical issues to do with the web standards being pretty crappy for complex GUI development.
You will, of course, be limited to the APIs available in standard Javascript. At the moment that means missing a lot of things that people might want to be able to do (or need to be able to do to write things that act like freestanding apps instead of websites). The way Mozilla tried to get around that was to add a lot of Firefox OS specific APIs and then claim that those APIs were part of 'the open web' because they were going to submit them for standardisation at some point in the future.
Mozilla seem to think that they are the sole arbiter of what the web standards are. The APIs they want to add are 'open' and 'standard' and anybody who is against adding those APIs is an entryist enemy of 'the open web'. APIs proposed by other vendors that Mozilla dislikes are an attack on the open web.
That's not how it works.
Things only become standards (formally or de facto) when all (or nearly all) of the big browser vendors support them. That's the definition of an open standards based platform - it's lowest common denominator. If you don't like that reality then you don't like open standards based platforms. There was little sign that the other browser vendors had any interest in adding a bunch of FirefoxOS specific APIs (just as how Mozilla have rejected various proposals from other vendors over the years).
You can argue that all this is very unfair. Why should Apple and Google and Microsoft have veto power over what APIs are available on FirefoxOS? But this is a trap of Mozilla's own choosing. It's the fundamental problem with the dream of a single platform standard for all user facing software. It creates a tight sandbox (both software architecturally and politically) that gives the big players veto power over everything. The idea of a single grand standard platform with perfect compatibility across all devices is certainly alluring, but it has huge downsides that nobody ever seems to really want to talk about. And that's before you get to the technical issues to do with the web standards being pretty crappy for complex GUI development.