Incorrect width for "overlay" div in Firefox - 17 pixels too large

When the lightbox opens using Firefix, the width of the div with the id "overlay" is 17 pixels too large - causing a horizontal scrollbar to appear in the browser window.

I looked into it and it seems that this is because the vertical scroll-bar for Firefox is 17 pixels wide, it seems lightbox is not taking this into account when calculating the width of "overlay" when being displayed. (perhaps this is an issue with Firefox not returning the correct "available" screen width?)

eg. My screen is 1440px wide. When I open a lightbox window, the width for "overlay" is calculated to be 1440px. Due to the fact that the screen I opened the lightbox from was vertically scrollable, there is also the right scroll-bar (which occupies 17pxs of width). So the lightbox is too large for the screen. When I change the width to 1423px using FireBug, it's the perfect size.

I have found these results for my site and also the demo site for the module. This only occurs using FireFox (I am using 3.5.3) and when the page opening the lightbox has a vertical scroll-bar.