According Wikipedia -Add ons (Firefox Extensions) are installable enhancements to the Mozilla Foundation’s projects. Add-ons allow the user to add or augment application features, use themes to their liking, and handle new types of content.
It makes web browsing so easy. Check the following add ons, i think they are best.
Adblock Plus: Adblock Plus allows you to control more in the website that you browse. This add ons remove online advertising to blocking all known malware domains. You can customize your filter.
Video DownloadHelper: Download Helper is a great add ons. You can easily download different videos in different format and can instantly convert them.
Aviary Screen Capture & Quick Launch (Talon): This add ons helps to quick capture the whole webpage or any portion of webpage.
Translator: This add ons you to translate any web page into nearly any language at the click of a button.
SEO Blogger: SEO Blogger is a super tool for Search engine optimization. By this add ons you can easily trace the keyword.
CookieCuller: By CookieCuller, keep your wanted cookies and it automatically delete the rest.
CustomizeGoogle: CustomizeGoogle is a Firefox extension that enhance Google search results by adding extra information (like links to Yahoo, Ask.com, MSN etc) and removing unwanted information (like ads and spam). All features are optional and easily configured
SEO & Website Analysis: SEO & Website Analysis is for fully analysis of a site.
Xmarks Sync: Xmarks Sync is a superb boolmarking add ons. You can keep your bookmarks, passwords and open tabs backed up and synchronized across computers and browsers.
Showcase: This add ons provides the way to manage your Firefox tabs and windows by showing them as thumbnails in a single window, tab or sidebar.
Flagfox: Flagfox helps to displays the country flag depicting the location of the current website’s server & provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more…
McAfee SiteAdvisor: SiteAdvisor software is an award-winning, free browser plug-in that provides safety advice about Websites when you need it – before you click on a risky site.
IE Tab: This is a great tools for web developer.
Social Media for Firefox: By this add ons you can easily bookmark your page in different social bookmarking site.
Restart Firefox: Use this add ons for easy and quick restart.