Div grid loses shape when zooming out in firefox

Hey

My grid is losing its form when I zoom in and out in firefox. Its basically a div container of 1000x1000px with 100 divs inside.

When I zoom out to a certain point it ends up 9 boxes wide and 12 down the last row only having 1 box on it.
I've set padding and margin to 0 and have no idea how to prevent this. No working sample online right now but this code below is ready to test needs no external files.

this is an interesting one for me so thanks for any help

<?php
	/* Loop 10 times to create a 100 square grid within a fixed area */
	//AD = 100 x 100
	
	$gridwidth = 10;
	$gridheight = 10;
	
	$adwidth = 100;
	$adheight = 100;
	
	$divwidth = $gridwidth * $adwidth + 20;
	$divheight = $gridheight * $adheight + 20;
	
	$adnumber = $gridwidth * $gridheight; //gets number of blocks required
?>
 
<!-- HTML IS BUILT FROM HERE ON -->
<div style="width:<?php echo $divwidth;?>px; height:<?php echo $divheight;?>px; border:solid 0px #000000; padding:0; margin:0;">
	<!-- GRID STARTS HERE -->
	<?php
		//       initial value   		
		for ( $counter = 1; $counter <= $adnumber; $counter += 1) 
		{	?>
			<!-- 1 BLOCK STARTS HERE -->
			<div style="width:<?php echo $adwidth;?>px; height:<?php echo $adheight;?>px; float:left; border:solid 1px #000000; padding:0; margin:0;">
				TEST
			</div>
			<!-- 1 BLACK ENDS HERE -->
			<?php  
		}
 
	?>
	<!-- GRID ENDS HERE -->
</div>

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