Firefox addon to have some movable lines visible on a page, for developing purposes

Question

I need to horizontally align with css two things that for some reason are positioned absolutely.

Because of this, I would really appreciate an addon that allows me to place some lines on the screen (which can be moved) to help me in this job, instead of taking screenshot and go to my image editing program to check if they are on the same height

Thanks for any suggestion

Using Firefox 10

Answer 1

I found the answer by myself: https://addons.mozilla.org/it/firefox/addon/measureit/

This addon works gracefully, really nice and helpful. What I was really missing was the word "ruler", I thought they were called guidelines

Answer 2

I can't think of a reason it needs to be a Firefox addon, but if not, you can Google for "on-screen ruler"and get a ton of results.