Not much to say. Simply forget about all the javascript wysiwyg editors

https://addons.mozilla.org/en-US/firefox/addon/4125

Firefox Extensions and plugins

Enhanced tab browsing.

This nice extension, will turn all the flash objects into a button, which you can press to load the animation.

Just what it says.

With the scrapbook extensions you can copy a page (or a part of a page!) in your computer, complete with images and everything. Extremely useful for archiving and searching .

This extensions allows you to remove any block (except embedded objects) from a page. It simply removes it from the layout.

Simply the most important tool if you are interested in web developing.
It can do anything, it's simply amazing ..

There are some firefox extensions that you simply must have.