As of firefox 3.0 each operating system came with it's own 'Default' Theme. You mentioned that
it's not a theme, it's the defualt Firefox icon set
However these are one in the same, Firefox's icon set is dictated by its theme. Ubuntu used to bundle an Ubuntu theme with the system install to give firefox a more cohesive look. Now however Ubuntu simply uses the default.
The mockups you are looking at are not the end product that was designed, for example the home tab as of 5.0 has yet to be implemented, and the progress line on the top of each tab does not exist.
The Linux mockups shown at the link provided are simply what a designer thought the browser should look like under all of Ubuntu's default bundled appearances.
In practice developers however have not implement the ability to dynamically load in the systems icon set. As firefox does not use gtk+ to theme its toolbars this would have been non-trivial to implement.
By going to the following
add-ons > appearance > Default 5.0 > More
You can see that this is the default theme by Mozilla (note that it can not be removed but other themes can be). The theme itself is called Tango, (the OSX them is called Firelight, the Windows theme is called Strata).
You can view the mockups made for Windows and OSX here
these are closer to reality than the Linux mockups but still not 100%.