It looks like a time zone error in the Firefox SpiderMonkey engine (most likely, it uses it in some library); in my experiments this only affects * nix OS, not Windows. (See below why it includes time zones.) It returns
Wed Dec 31 1969 23:00:00 GMT + 0000 (BST)
... when, of course, we (@wilsonpage, I and millions of other people in the UK) are no longer in British Daylight Saving (and when we are in summer, it is GMT + 0100, not GMT-0100, so anyway for local time, it would be unwise to lag behind UTC). (Note: Chrome also shows “BST”, but it has the correct date / time.) (Note for Americans: you’re used to “xST”, which means “x standard time” [as opposed to “x daylight time"], but here BST means "British summer time", for example, summer time. When we are not in summer time, we are in GMT.)
I see this behavior in Firefox 30 on Linux Mint 16 (apparently the last in the package system) and Firefox 33 on the same system (just downloaded and installed directly from Mozilla); I do not see it in Firefox 33 on Windows 8.1.
@wilsonpage has confirmed that he uses OS X and that he sees the same time as me (including the "BST" part).
result.textContent = new Date(1970, 0, 1).toString();
<div id=result></div>
The reason that they call at this time (and apparently initiates an error) indicates that the values that you give to this version of the Date constructor are interpreted as UTC, but getFullYear returns its value in local time.