I have gone with Firefox after the recent revolution of Opera. These are the extensions, that have helped me re-recreate exact or approximate functions from Opera. Suggest if something's missing.
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.
The first and only download manager/accelerator built inside Firefox!
Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
LastPass, an award-winning password manager, saves your passwords and gives you secure access from every computer and mobile device.
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
A customizable mouse gestures extension which enables to execute various commands and user scripts with six types of gestures.
View HTTP headers of a page and while browsing.
Just another speed dial extension. Fast and easy to use.
Make the resources behind container pages easily downloadable by DownThemAll!
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.