It's kinda funny how many Mac users are quick to say Firefox isn't very Mac OS X-like.
On Windows, Firefox is actually one of the more OS X-like programs.
Look at the Options/Preferences window.
Older Firefox users will remember how different it used to be before version 1.5.
From 1.5 onwards, it adopted the look of a typical OS X app's Pref window.
Also, how the toolbars work.
Usually, on Windows, you move the toolbars themselves, and the toolbar items aren't movable, except in some cases, if there's a "Customize" option.
If you look at how customizing toolbars work in Firefox, that's almost exactly like typical OS X apps.
And I also find Firefox's design to be Mac-like in the sense that everything is very consistent, especially when it comes to the details.
For instance, middle-clicking and Ctrl/Command-clicking will always open in a new tab.
It works for links in pages, bookmarks on the Bookmarks Toolbar or in the Bookmarks Sidebar or in the Bookmark Manager, items in the History menu or in the History Sidebar, the View Image context menu item, and even the Home button!
(and in the next Firefox release, it'll even apply to the Location Bar dropdown's items)
Most Windows apps aren't this consistent and don't pay this much attention to the little details. (Try the middle-clicking/Ctrl-clicking thing in IE7.)
However, Firefox is meant to be cross-platform, and thus, Firefox for Windows and for OS X are nearly identical in every aspect.
As a result, there are compromises on both sides. (However, it's likely that you'll notice them much more on OS X than on Windows. On Windows, you probably wouldn't recognize any "un-Windows-like" elements, because there aren't many standards when it comes to program look & feel and behavior on Windows.)
So while it's true that Firefox is a relatively OS X-like program compared to Windows programs,
it's also true that it's not as OS X-like as other OS X apps.
But personally, for me, this is an acceptable compromise for the consistency of the design, the fact that in both OS X and Windows, I can use the same browser, one that is somewhere between an OS X app and a Windows program, and of course, the multitude of extensions and themes, etc.
Also, ironically enough, speaking of the middle-click thing,
there is currently a bug in Camino: Command-clicking a bookmark will open it in a new tab... but middle-clicking a bookmark won't do anything.
The developers have said that they're aware of it, and it'll be fixed... eventually.