Firefox Text Issues

[Ropetin Again]

I /may/ have come across a bug in Firefox but I'd like to confirm with
you guys first whether I have, or maybe its just a 'feature' I was not
aware of.  If it is a bug I'd like to know am I better to post it as a
bug with the Ubuntu bug list or the Firefox bug list.

In Firefox, the gap between certain letters, in certain situations
changes when surrounding text is highlighted.

Take these 2 images;

http://www.mrboyonline.com/images/linux/ss1.png

http://www.mrboyonline.com/images/linux/ss2.png

The distance between the 2 S letters in 'password' definitely changes
when the word 'generated' is hightlighted.  Other words and letters show
the same thing.