CSS Float Edit to Twenty Ten Theme Not Recognized in Firefox

[ Moderator Note: Please post code or markup snippets between backticks or use the code button. ]

I am a WordPress and HTML/CSS newbie. I have been tweaking the CSS on the Twenty Ten theme to customize and learn the basics. All my CSS edits have worked great and then I tried to add some new lines of CSS to the style sheet to float some images into a grid pattern. IE and Chrome look great, but Firefox doesn’t work. I used firebug to try to figure out the problem – I can’t even find the added CSS from within Firefox. If I use Firebug lite from Chrome, I can see, validate CSS. I am at a loss….help. Below is html and added css. Thanks in advance for any help or suggestions.

HTML:

/*
Hopefully this will be my portfolio landing page....if I can get it to work.
<div class="threerow"><a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>
</div>

<div class="threerow">
<a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>

</div>
<div class="threerow">
<a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>

</div>
<div class="threerow">
<a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>

</div>

<div class="threerow"><a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>
</div>

<div class="threerow"><a href="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg"><img class="alignnone size-full wp-image-98" title="workthumbnail" src="https://printandpixelcreative.com/blog/wp-content/uploads/2012/06/workthumbnail.jpg" alt="" width="175" height="150" /></a>
<p class="worktitle">title of work</p>
</div>
*/

CSS (only the new lines I added to the style sheet):

/*
.threerow {
float: left;
margin-right: 25px;
margin-bottom: 25px;
border: 2px;
border-color: #E8E8E8;
width: 175px;}

.worktitle .worktitle a {
color: #C8C8C8;
font-family: sans-serif;
font-size: 10px;
line-height: .5em;
text-decoration: none;
width: 200px;}

.worktitle a:hover {
color: #A0A0A0;}
*/