There’s a cool Firefox add-in that puts rulers and guides in the browser window, so you can measure things just like in Photoshop, Flash, Quark and most other graphics applications. The add-in is called foxGuide and you can download and install it from Mozilla here. Just be aware that it’s currently a beta release. Like most other add-ins, it’s free, and the developers don’t even seem to be asking for donations. I’m running foxGuide in Firefox 4 beta 9 without a problem.
Once foxGuide is installed, activate it from the pop-up menu when right-clicking the screen:
You can then drag as many horizontal and vertical guidelines on the screen as you want. Remove one at a time by dragging it off the screen, or remove them all by right-clicking and choosing Reset Guides. Or just refresh the browser.
The only minor glitch is that because the rulers lie on top of the screen, they take up a small amount of space (rather than pushing the screen away), so if you want to measure something right at the edge it can take some fiddling. But that isn’t terrible; just make the rulers transparent to see what’s behind them. You probably also want to fix the rulers in place, otherwise they’ll scroll out of view when you scroll the page. You control the preferences with a small control panel in the lower, right corner of the screen.