There are a few Firefox Add-Ons that I can't live without, Here's my short list (IE the absolute essentials. Obviously, I have lots more, but I can give or take them):

AdBlock Plus - This is the one everybody in the world has. Websites look SO much different when I see them with the actual ads on them now (ie at a friends house w/ only internet explorer)

Repagination - Loads all pages in a list on a single page... So when I'm browsing the books at gigapedia.org, I can load 20+ pages onto 1, and scroll right through 300-500 books without having to reload a page. Huge timesaver. (For Firefox 3, get the Non-Official version here > Repagination Firefox 3)

FireFTP right in the browser - no more wasted effort or time trying to look for FTP software and no need to go out of the browser

All in one gestures - great time saving tool once you learn a few basic mouse gestures(takes only 5 mins).

Web Developer - helps with website development, very helpful while designing pages, scrutinizing existing sites.

PicLens - great tool for:
- searching for images
- browsing through images on websites (google yahoo facebook myspace)
- searching youtube
- video news

Fun and easy to use



IE Tab - awesome plugin that Enables you to use the embedded IE engine within Mozilla/Firefox. With a single click, you can test for cross-browser compatibility
AI Roboform (saves my passwords and logon info...good for paying a lot of bills online)
DNSStuff Tools-Bar

Gladder - great proxy tool. You forget it's there.

Video Download Helper - to save videos from Youtube etc on my HD

PDF Download - to avoid the annoying load time of online PDFs (personal pet peeve)

Tempomail - generate a temporary email on the fly with one right-click

Aaron Wall's Seobook has some good tools which are compatible with firefox:
SEO Book Keyword Suggestion Tool
SEO for Firefox
Search Engine Ranking Checker
Search Analytics Tools
Competitive Research Tools
PPC Tools
Link Analysis Tools



Live HTTP Headers - Powerful behind-the-scene info about web sites/pages.

NoScript - protects from malicious scripts

Enjoy

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