Developer tool is an add-on of Mozilla which helps to debug the elements in a web page. You can get the tool from Add-ons (go to Add-ons and install web developer Tools (Add-ons > install web developer tools). On complete installing the Add-on, you can see a menu bar with items like disable, cookies, images, miscellaneous etc. You can use these items to measure, re-size, disable images, view source code, responsive view etc.