No, Shiira is not an Apple product.
Essentially, since Safari became the default web browser on Mac OS X, replacing IE5
for Mac, a number of other web browsers have started to appear, based upon the
same HTML rendering engine, called Apple WebKit, which in turn, is based
on the rendering engine found in KDE's Konqueror web browser.
The other 3rd party browsers for Mac OS X built upon the Apple WebKit rendering
engine I'm currently aware of are - OmniWeb 5.0 - TrailBlazer - and now Shiira.
I think what might happen eventually, is that these browsers will probably just use
the copy of the Apple WebKit rendering engine included with whichever version
of the Safari browser is installed on the computer, in much the same way that
some browsers made for Windows, tend to rely on the Trident rendering
engine from whichever version of IE is currently installed.
These types of browsers are often referred to as "wraparounds" or "shells". Although
the aformentioned Apple WebKIt based browsers each currently bundle their own
copy of the rendering engine, they could eventually become "wraparounds" or
"shells" relying on a copy of Safari to be already installed on the computer.
If anything, they would most likely do this mainly to keep their own download
filesizes to a minimum, and also assume all users of Mac OS X have a current
(or recent) version of Safari installed, in pretty much the same way that the
makers of Avant Browser, and MyIE2 (amongst others) rely on Windows
users having a current (or recent version) of IE already installed.
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.6) Gecko/20040113
SeaMonkey = Swiss Army Knife: It's versatile, reliable, and contains useful tools.
Windows Internet Explorer = Old Swiss Cheese: Full of holes, and it stinks!