I have encountered a very weird thing in firefox and opera. If you goto www.xboxtoday.ca and look at the google ad at the very top of the page under the logo, there is a very little white line underneath the google ad. It is not there in IE though. If anyone knows how I can fix this, can you please let me know? Thanks.
I have temporarily fixed it by setting the bgcolor of the cell to the same as the adsense border color. Would still be nice to know why it did this in the first place.
I have a similar problem on a site I'm putting together. I added this doctype <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> to allow centering in IE6, but it broke my layout in Opera and FF. There are now white lines appearing between the tables on the site. If you figured your problem out, please let me know how you solved it.
Like I said for now, all I did was do the background colour of the TD(Cell) the same colour as the adsense border colour and the white line went away. I have not figured out a way to fix the line showing without setting the cell background colour.
I had the same problem of Firefox showing a white line where IE didn't. Someone told me to put " ul {margin: 0; padding: 0;} " (no quotes) in my CSS and that fixed it. Hopefully it will work for you too.