Photoshop-Like Guides In Firefox – foxGuide
foxGuide is a firefox extension that can display both horizontal and vertical guides on web pages.
The guides can be moved or removed just like the ones in Photoshop. Also, you can set the transparency and color of them for a perfect fit.
The extension has a memory which remembers the guides created per website. Even if the browser is closed or the URL is changed, the guide for that web page is stored.
foxGuide also works well for pages that have huge width/heights (supports up to 10,000px).