This extension is absolutely invaluable. It helps me figure out so many elements on a screen as I design websites.

Does not let you adust the dimensions once you take your finger off of the key, like you can do when cropping a photo, for instance. If the cursor starts slightly off of where you want it, you have to start all over again.

Perfeito. Facilita demais o meu trabalho.

Doesn't help much, because you can't move the selected area or correct/change its borders after selection. That means, if you did not hit the area to be measured pixel-exactly at the first go, you have to repeat the measurement until it fits as desired sometime.

Size in Pixels makes no sense with me. I need size in cm/inches, something eas for you to do if you use the screen size, resolution and pixels. Why should I do it, when a simple add-on to the add-on can easily do it. Until change implemete I'll discard "Measure it". Even with "paint.net" I can easier get/adjust size for optical purposees.

Good. it definitely does what it proposes. But it should have a percentage option. Percentage relative to the window in width and height.

Mas que buena. Muy útil sobre todo para desarrolladores y diseñadores que quieren sacar medidas en la pantalla.

De uso muy fácil. ¡¡¡100% recomendada!!!

Unfortunately, it doesn't work on all websites (I'm not talking about *.mozilla.org).

Installed in Mozilla and Chrome, now works correctly on a site outside Mozilla, which I havent tried before. Sorry