https://hacks.mozilla.org/2017/08/introducing-the-extension-compatibility-tester/?
Firebug
Colorzilla
Measure It
Abduction (screen capture): Right click on the page and select "Save Page As Image". You will then have to select appropriate region to capture and then click on appropriate control.
Screengrab (another screen capture)
New Tab Homepage: Load your homepage whenever you open a new tab
Dust-Me Selectors - is development tool that scans your website to find unused CSS selectors.
https://addons.mozilla.org/en-us/firefox/addon/alertbox/ - Track changes in web pages and feeds. Get alerts for important changes.
FlashGet / FlashGot
What is the purpose of Distill?
Distill offers ability to track webpages and feeds for changes, get email & SMS alerts. Instant alerts notify you of changes wherever you are. There are rich types of actions to receive change notifications that you can personalize for each monitor. We can get SMS alert, or Email message, or we can integrate Distill with external application using webhooks. Once installed, Distill show up as an icon in the top tool bar, but we can also directly access the web interface via https://distill.io/
