MeasureIt
MeasureIt - see the official homepage here and the listing in addons.mozilla.org here - is quite a useful add-on for checking the width, height, alignment of elements in webpages. It is a must have for developers, (although I haven’t found that use to this extension yet; I am not quite into developing yet. 
). It is also useful to me in forums, to check whether my signatures are too big than the allwoed size. MeasureIt is also useful for checking the alignment of different elements in a webpage. Check out how MeasureIt comes in handy for me in forums :-
To turn it on, just click on the MeasureIt button 
Besides, you can also drag and drop the measurement boxes; after you take the measurement, just hover the cursor above the box, and a move cursor appears. You can also “nudge” the measurement box by small increments; use CTRL+Arrow key to move it by 1 pixel, and use CTRL+SHIFT+arrow key to move it by five pixels. After you have done, just click on the little red X to close that measurement box and take another measurement. And to turn it off, just click on the button
Also this extension has a problem - flash elements are disabled when this extension is activated - this still is one of my can’t-live-without Firefox extensions.
4.5 out of 5 stars!
[…] to think of all the trouble I had to go through to show a snapshot of the working of MeasureIt in my review..screen capture, copying it into Photoshop, cropping it, saving it,uploading it, and then linking […]
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Well, when you need to use MeasureIt but not Flash Elements, turn it ON, and OFF it when you are not using. Unless you need to take the measurements of Flash videos, images and stuff, then I think you may have a big headache.
In my opinion, Webmaster can benefit alot from this extension. Imagine if you need to maximise your post area and you would normally need to use the maximum image size (width) that you wish to include into your blog post. With MeasureIt, it would be useful for me since I can measure the width of my blog post and determine how wide I want my image to be, smart right?
This would prevent overflow of image size which may result in messing up the blog appearance.
Glad to see you liked this extension. Hope you enjoyed my review as well.
I have problems with Firefox 3.0
What sort of problems? Maybe I can help you.
This looks good. I’ve sometimes tried coding html using absolute position div elements, and it can get pretty difficult to know how things are going to line up. I think this might help, thanks :).
I have been using a few good firefox extensions recently and this one looks like a useful tool. Normally I have to resort to making images and doing screen captures to get DIV sizes right, but with MeasureIt I can see that it doing screen captures will be a thing of the past.