HTML5 CSS3 WordPress

can you post more details such as a link to the site to illustrate how the posts are appearing?

seems to be a problem with clearing floats;

try to add this to style.css:

article:after { content: '.'; float: none; clear: both; height: 0; visibility: hidden; }

Which theme is it you’re using?

style.css of your WordPress theme.

does not matter, where exactly you add the new style (obviously not violating the css syntax, not into any existing media queries or print styles, …).

It doesn’t seem to be in your style.css file.

clear your browser cache – press CTRL F5 or press ‘reload’ a few times…

this is what I see in FF20:

http://imageshack.us/photo/my-images/845/screenshotge.jpg/

what do you expect to see?

Can you install your theme again?
There a HTML element

<div class="container">

That is missing from your HTML structure.