First a disclaimer, I'm writing about something I have no personal
experience of - I don't even know JavaScript. Any knowledge I have
comes from reading various posts here in the old and new Seamonkey groups.
Google has added new functions and operators to JavaScript. These have
been implemented in Chrome and propagated to the various Chrome-based
browsers.
This has forced Firefox to follow suit. Seamonkey used to be very close
to Firefox, but Firefox have made so many changes - a lot of them on
things which would have been better left alone - that the code-bases
have diverged, an ongoing process. Adapting these Firefox updates to
Seamonkey is not as trivial as one would have hoped, and there are only
so many hours in a day.
Firefox is open source and I believe Chrome is as well, otherwise we
would not be seeing all these Chrome-derived browsers.
Are they secure? One would hope that they are, that is at least
partially down to the implementations.
Are they optimisations? Google would presumably argue that they are,
although some of them appear to have been made "because they can".
Back to the word "Secure". Once upon a time PDFs were considered
"safe". Then Adobe added all these wonderful features - such as
unvalidated database access - and we arrived at the current situation
where PDFs have to be treated like Word documents. Adobe no longer have
an Acrobat Reader for Linux, there are a whole bunch of other projects
to fill that need but none of those programs implement the full range of
features, some of them are simply too dangerous.