I'm not sure I'm following clearly... if you pin the Firefox shortcut to your taskbar then that's always where it will stay, exactly where you put it. If you open multiple instances (right-click and select Mozilla Firefox at the top of the popup, to open another window instance) they will just be right next to each other.
For example, I have Firefox's shortcut pinned to my taskbar as the third icon to the right of the Start orb. When I open the first instance, I get the icon enlarged into a rectangle. If i right-click on that enlarged icon and select Mozilla Firefox from the popup menu, I get a second instance and a second rectangle immediately to the right of the first rectangle. Repeat again... and I now have three consecutive icons.
It looks like this:
You don't have to only use the "pin to taskbar" to get a shortcut placed onto the taskbar, since that method adds it to the end of the line of current icons on the taskbar.
Instead, you can just carefully drag-and-drop a shortcut from the desktop, down over that place on the taskbar where you want it to be... say between two particular existing shortcut icons. Keep the left-button on the mouse held down as you position the dragged desktop shortcut into the space between the two existing icons where you want the third shortcut to go. Windows will then SPREAD out the two icons, making room for your new third icon in between them.
Now you can release the left-button, and presto! Your new shortcut is placed exactly where you wanted it, BETWEEN the two existing icons.
That's how I got my Firefox icon on the left side of my taskbar, because I use it constantly. And by having it on the taskbar (like the WinXP quicklaunch bar) it will always open exactly there into a rectangle, when you launch the first instance. And each subsequent instance will simply be a consecutive rectangle right next to the previous group. Passing the mouse over any of them presents the Aero preview of them all.
Personally, I prefer mulitple tabs to window instances, but it's just personal preference.