While this may be an eclectic mix of add-ons, I find them to be necessities in making the perfect Firefox experience. Updated to reflect my needs for Firefox 34.0.5
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
The easy way to download and convert Web videos from hundreds of YouTube-like sites.
Displays a country flag depicting the location of the current website's server and provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more...
This extension allows you to export your saved passwords and disabled login hosts using XML or CSV files that can be imported later.
Make your bookmarks bar better. Roomy Bookmarks Toolbar can: hide bookmarks/folders name(dock like), multirow, autohide and many another features(more than 20).
Customize your bookmarks bar using one add-on.
Make any pages' search functionality available in the Search Bar (or "search box")...
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
Returns the 'Properties' menu item that was removed in Firefox 3.6 to the context menu.