Here are some helpful Firefox add-ons recommended by Smartlab Software. We use these nearly every day in our web site development.
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This add-on blocks all Flash, Shockwave, and Authorware content from loading. Placeholders are displayed instead of Flash. To show the Flash, click on the placeholder and it will appear.
Flashblock saves a lot of browser load time and hides a lot of ads.
Checks links on a web page. Colors the links green if they are ok.
McAfee Site Advisor
Site advisor gives safety information whether a site has issues with it, such as spyware downloads.
I use this one all the time. Session manager saves the previous ten sessions which can be reinstated. It also can recover crashed sessions.
Point your cursor to a color on the screen and ColorZilla will tell you the RGB and hex value of it.
Lynx Viewer Tool
View a web page as a search engine sees it. All images and other superfluous baggage are stripped out so only text is shown.
Tab Mix Plus
Manages tabs such as duplication, closing, and more.
Analyzes a web page for factors that contribute to slowing it down.