The support folder only contains DirectX redistributables and isn't used by rFactor. d3d9.dll is a part of Direct3D, some add-on developers use it to hook into Direct3D to overlay graphics onto the screen, like TV Style and rFDynHud do. The "FPS Fix" add-on is another example. I think there are at least two variants of it where Wombat's is one, and the other one is by the guy who wrote RealFeel, if I'm not mistaken. I'm not exactly sure what those FPS Fix things do, but I vaguely remember that it has something to do with utilizing video memory buffers to a greater extent than what rFactor does by default.
@Jouni: Is your DirectX up to date? Not sure it would matter, but if you haven't updated it for a while, it might be worth a shot.