Here is a good example of using Firefox Responsive Design View, DOM and Style Inspector to debug web design issues. Noticed this problem with Mozilla The Den blog's site description while browsing the web with Firefox Mobile browser. Fired up my laptop to check this design issue with Firefox Responsive Design View. Then I used the DOM and Style Inspector to zero in on the CSS rule that adds this extra padding (padding-right: 340px;). Voila! It is time to send this patch to the developer.
Did a screen capture of the whole process. Don't see the embedded video, you can watch it on YouTube.