DevBoi: Firefox Sidebar For Web-Dev References
The sidebar is very handy for any developer as you can reach it with a simple "CTRL+F9", find any function or property, see the browsers it supports & more.
Standard installation supports:
- HTML 4.01/XHTML 1.0,
- CSS 2.1,
- DOM 2,
Support for other languages can be downloaded from here.
You can also go the original website for the add-on for downloading it without registering.