An ongoing comprehensive user.js template for configuring and hardening Firefox privacy, security and anti-fingerprinting - ghacksuserjs/ghacks-user.js
Firefox Extension for web developers/designers as well as casual users "As you glide the mouse over the page, you will see a red rectangle framing each element under the cursor."
Firefox Extension, simple tool that allows you to highlight all instances (within a webpage) of a word, several words, or a phrase that you currently have selected. Right click anywhere on the page and choose "Clear Highlighting" to reset the page.