I have tried various Firefox add-ons over the years, but the only ones I have kept are these:

NoScript

Only allows scripts to run on web sites that you trust. It also ends up working like an ad blocker most of the time, so Adblock Plus usually isn't needed.

Tab Mix Plus

Gives you more control over tabs. You tell the browser how you want to use tabs instead of letting the browser tell you what to do.

Download Statusbar

Shows info about downloads in the statusbar and will automatically scan anything downloaded with your anti-virus software if your software allows it (Avast is one that allows it).

DownloadHelper

Allows you to download flash videos from many web sites, including YouTube. Videos from YouTube can also be saved as .mp4 files for use with the QuickTime Player.

Googlebar Lite

Besides the usual search stuff, it's the fastest way to search a web site. Just type something in the search box and click the Search this Site button and you're all set. It's also a faster and easier way to find a word or phrase on a web page. Just type your word (or phrase in quotes), then click on the newly created button or buttons to find the word or phrase you're looking for. If you come to a page through a Google search, the word buttons will already be there for you to click on. It's like a search supercharger. You spend less time searching and more time finding.

Menu Editor

Customize application menus. You can get rid of the crap you don't use from the right-click menu and more.

DownThemAll!

It's great if you need to download a bunch of files. It will download two files at a time until the job is done.

The following two add-ons are only useful if you have a web site:

SearchStatus

When you visit your pages, this displays the Google PageRank, Alexa rank, and more in the statusbar. Keeps you informed as you browse your web site. If your PageRank drops, Google is probably punishing you for some imaginary sin you've committed against the search engine gods. Once you see that your PageRank has dropped, you can kill a chicken and say a prayer to the giant space turtle and see if it helps get your PageRank back up where it used to be.

Link Evaluator

Checks all links on a page. Find those dead links without needing to click on hundreds of links by hand. It can save you a lot of time and energy.

Random Terrain

_