Firefox

Firefox Plugins

Flash Aid - fixes Adobe Flash problems.
Restart Firefox - adds File->Restart.
Restartless Restart - adds the menu File->Restart
Rss Icon in Awesomebar - adds the rss icon
Session Manager - saves automatically your sessions
Scrapbook+ - save web pages offline and organize them. Useful when on the way without internet connection.
Tree Style Tab - moves the tabs on the left side. Much easier to navigate.

Adblock Plus - remove Ads.
Download YouTube Videos as MP4 - Requires Greasemonkey.
Element Hiding Helper for Adblock Plus - perfect for hiding undesired elements from websites. Shortcut: Ctrl+Shift+F3. In some cases better than "Remove It Permanently". (Requires Adblock Plus).
Find and Replace - you can find and replace text on your page. Especially handy when editing the Wiki!
Boomerang for Gmail - Send an email later, follow up reminders, remind you if you don't hear back.
Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Gmelius - Gmail: ad remover, optimized layout.
Remove It Permanently - Permanently hide content from web pages using the Context Menu.
Screengrab - click it, select the region, save the screenshot - very simple and quick.
Shareaholic - share interesting links using Facebook, Twitter, Google Mail, Reader, Bookmarks, Evernote, Bitly, StumbleUpon, and more.
Textarea Backup with expiry - Backup textareas content for accidental losses. Automatically remove abandoned backups after preferred time span (Needs Greasemonkey).
Video DownloadHelper - download videos from youtube and convert them.
Wired-Marker - highlight website texts, you can define your own shortcut from the properties of every color.

Google Privacy - Removes tracking links from Google, YouTube, Yahoo!, Bing and Facebook.
Ghostery - Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
HTTPS Everywhere - encrypts your communications with many major websites, making your browsing more secure