Historically, SpiderMonkey’s JSON implementation has accepted input containing trailing commas:

JSON.parse('[1, 2, 3, ]');
JSON.parse('{ "1": 2, }");

We did so because the JSON RFC permitted implementations to accept extensions, and trailing commas are nice for humans to be able to use. The down side is that accepting extra syntax like this makes interoperability harder: implementations which don’t implement the same extension, for reasons every bit as valid as those of implementations allowing the extension, are disadvantaged. ES5 weighed these concerns and chose to precisely specify permissible JSON syntax, putting everyone on the same page: trailing commas are not permitted. Therefore, the examples above should throw a SyntaxError per ES5.

SpiderMonkey has now been changed to conform to ES5 on this point: trailing commas are syntax errors. If you still need to accept trailing commas, you should use a custom implementation that accepts them — but best would be for you to adjust the processes that produce JSON strings including trailing commas to not include them. (If you are an extension or are in privileged code, for the moment you can use nsIJSON.legacyDecode to continue to accept trailing commas. However, note that we have added it only to accommodate legacy code in the process of being updated to no longer generate faulty input, and it will be removed sometime in the future.)

You can experiment with a version of Firefox with this change by downloading a TraceMonkey branch nightly; this change should also make its way into mozilla-central nightlies shortly, if you’d rather stick to trunk builds. (Don’t forget to use the profile manager if you want to keep the settings you use with your primary Firefox installation pristine.)