That depends on what you want changed. Take a look at this add-on,

NoSquint {web link} NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.

freemds..

i want to change websites layoit for example background image etc

style.org site ?? What does that have to do with modifying Firefox?

Are you referring to the Stylish extension? Pre-made UserStyles for Stylish are available here - https://userstyles.org/

And you can use a userContent.css file in Firefox to create your own page "styles" without using Stylish. http://kb.mozillazine.org/UserContent.css But there's a large learning curve involved when doing your own userContent.css file.