Open/Save local (JSON) file from JavaScript >> IE/Firefox

I'm very new to JS and I'm doing a small html page that -for now- will be ran locally. I have a string in JSON format which I need to be able to store/load as a file on the hard drive.

To be able to store the string, I've got this to work on Firefox:

function saveJSON() {
    var obj = {name:'John', max:100};
    window.open( "data:text/json;charset=utf-8," + escape(JSON.stringify(obj)))
}

However, it only works on FF, and I need to be able to do it with Internet Explorer also. I've read some stuff about using ActiveX, but I haven't found any example on how to do it.

Should I try and use ActiveX, or is there a better HTML/JS way to save the file which works for both browsers?

The second problem is loading the JSON file. I've found that once loaded, I can turn it into a JSON var with JSON.parse. But I have no idea how to load a selected JSON file. I have an

<input type=file id="filePath"> 

to get the file path (though it returns different things in both browsers), and I would like to be able to do something like

var a = loadFile(filePath.value)

Any suggestions on how to do it? I'm really stuck here and would greatly appreciate any help.

Thanks.