Sage is a lightweight RSS and Atom feed reader extension for Mozilla Firefox.
It’s got a lot of what you need and not much of what you don’t.
Sage - RSS and Atom feed reader extension for Firefox (...)

TwitterFox is a Firefox extension that notifies you of your friends’ tweets of Twitter. (former name is TwitterNotifier)

This extension adds a tiny icon on the status bar that notifies you when your friends update their tweets.
Also it has a small text input field to update your tweets.
TwitterFox - Firefox extension for Twitter (...)

Password Exporter is an extension for Mozilla Firefox and Thunderbird that allows you to export and import your saved passwords and disabled login hosts from Password Manager.

The logins will be exported to an XML (Extensible Markup Language) or CSV (Comma Separated Values) file and can then be imported into another application or computer that has Password Exporter installed.
Password Exporter (...)

This splits the content area of the browser window as you like.

This splits the content area of the browser window as you like. It will help you in various cases. For example, to compare multiple webpages, to show a calendar always, and so on.

There is some ways to split window.

• Choose the “Split Browser to” menu in the context menu.
• Choose the “Load in Split Browser” menu in the context menu on link.
• Click popup-button on top/bottom/left/right edges of the content area.
• Drop links, bookmarks, etc. to popup-button on top/bottom/left/right edges of the content area while dragging.

You can split browser infinitely, as many as your RAM and your screen allow. Enjoy split and split to your heart’s content!

Drag and drop is available to load links, bookmarks, and so on, into split browser. If you drag the toolbar of split browser, you can load the page in another split browser.

Split browsers can be closed by their closebox. “File” > “Close All Split Browser” closes all of them at once.

You can tile all of tabs as split browsers, and, gather split browsers to tabs. If the Multiple Tab Handler ( https://addons.mozilla.org/firefox/addon/4838 ) is available, you can tile tabs with selection.

This includes tabbed browsing feature. You can open new tab in split browsers, by “New Tab” menu in the context menu in the toolbar of split browsers, or middle click links in them. (But this feature doesn’t work with TBE or TMP. They are designed for environments without multiple “tabbrowser” widget.)

Status of split browsers are saved, and automatically restored on the next startup.
Split Browser (...)

Detects Duplicate Bookmarks when bookmarks are added and specify where is the previous URL. You can also search and delete duplicates URL already in your Bookmarks.

If you have a lot of bookmarks this extension is for you !

If you think it is difficult to find a bookmark within all of them then you need this extension. BookmarkDD add possibility to search string within bookmark’s location and bookmark’s description field.
See screenshots for list of options:
http://bookmarkdd.mozdev.org/screenshots.html

You can customize this extension with its configuration panel.
Bookmark Duplicate Detector (...)

Torrent Finder Toolbar is a torrent search toolbar for Firefox, which enables users to search over 170 top torrent sites and trackers from their Firefox browser the same way they used to search using www.torrent-finder.com form.

It allows users to search a single site, search all sites on one page, or open the sites in Firefox tabs.
A lot of accessible tools have been added to the Toolbar to make it easy to find your torrent like ‘keyword highlighter’, ’select toggle’ to toggle between search engines by category, and ‘toggle by first letter’ to easily find the search engine you need.
Torrent Finder Toolbar (...)

deliGoo is an extension for Firefox (2.0+), which is built into your browser in the form of an additional button on the tools’ panel.

deliGoo searches on the sites, indicated in your del.icio.us bookmarks. With its help you can find the necessary page according to any phrase or word, which it contains.

And one more thing – deliGoo can create on your demand custom search engines on any del.icio.us tag.
deliGoo - Delicious (del.icio.us) Search Engine (...)

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.
CustomizeGoogle (...)

FoxyTunes lets you control almost any media player and find lyrics, covers, videos, bios and much more with a click right from your browser.

Supports WinAmp, iTunes, Yahoo Music Engine, Pandora, foobar2000, Windows Media Player, Xbox Media Center, Musicmatch, Quintessential, J. River, jetAudio, XMPlay, MediaMonkey, Media Player Classic, Sonique, wxMusik, Real Player, XMMS, Noatun, Juk, Amarok, Music Player Daemon, Rhythmbox and many other players.
FoxyTunes (...)

Orbit Downloader, leader of download manager revolution, is devoted to new generation web (web2.0) downloading, such as video/music/streaming media from Myspace, YouTube, Imeem, Pandora, Rapidshare, support RTMP. And to make general downloading easier and faster.

Orbit Downloader supports HTTP/FTP/HTTPS/RTSP/MMS/RTMP protocols and provides a total solution to download rich media. So you can download music and video from social music/video websites including Pandora and YouTube more simply and easily.
Orbit Downloader (...)