Most Firefox add-ons enhance the browser's core feature set, provide convenient Web browsing tools, or plug in the proverbial bells and whistles (e-mail notifiers, current weather data or cosmetic makeovers).
But there are others that could be considered applications on their own. Some of these "add-on apps" work well enough to replace a standalone desktop application. Here are seven add-ons that transform Firefox into a fully functioning application.
This add-on app works so easily you might decide to forgo a standalone FTP file manager program. FireFTP is compatible with versions of Firefox for Windows, OS X and Linux (which is worth noting because the same add-on recognizes the file structures of these different operating systems).
FireFTP opens up like a normal tabbed page. Your local computer's file directory is shown in a pane on the left side, and the file structure of the remote server on the right. Simply click on whichever files you want to move, and select a direction arrow between the two panes to initiate a transfer (for example clicking the right-pointing arrow will upload files to the server).
Online Music Player 184.108.40.206
It's debatable if Online Music Player counts as a "pure" add-on for Firefox, since it requires Flash to be installed on the browser. But it's hard to ignore that this is a very capable MP3 player matching the basic features and functionality of a standalone application: it has an easy-to-use interface, but also includes playlist management, and lyrics search.
Online Music Player resides along Firefox's status bar, where its player controls, and the title of the song playing are displayed. It can play your MP3 files stored locally on your computer, as well as, conveniently, MP3s you find on the Web (by you right-clicking on the link to the file and selecting "Play It").
Pencil was designed for users to create business diagrams, and for developers to quickly throw together GUI mock-ups. (It comes with a selection of graphics illustrating common user interface widgets.) But its toolset allows for more artistic purposes, as you can make clip art with it. You can export your completed artwork as PNG, PDF or HTML files, or even as Microsoft Word or OpenOffice.org documents.
Like Animat, Pencil opens a new window and runs within it, and, thus, can work as its own application when you close the first open instance of Firefox.
Simple Mail 2.74
Most people use Webmail nowadays for their e-mailing needs. But if you require accessing your e-mail through a POP3 or IMAP server (or you would like to save your e-mails to your local computer) and you'd rather not fuss with a standalone program, this add-on transforms Firefox into a full-fledged e-mail client.
Simple Mail opens as a tabbed page within Firefox. It features the ability to handle multiple POP3 and IMAP accounts, e-mail composing with an editor that lets you use various fonts and colors, an address book, and inbox message filtering and other customizable functions. Like FireFTP, Simple Mail works well enough that you might choose to regularly use it in lieu of a standalone counterpart.
Like Animat and Pencil, SVG-edit pops open its own window and can, thus, function like a typical desktop program. This add-on app features standard drawing tools: freehand, point-to-point line, and rectangle and circle/ellipse drawing. You can incorporate text into your design. As it's meant for creating vectorized images, SVG-edit's primary editing tools allow you to manipulate (such as move, resize) and extract the layered components of your art.
You can only save your work as an SVG file. (Currently, you cannot export it into other image formats.) Yet SVG-edit provides the essential tools you'll need for creating vector graphics, and it runs quite nimbly.
There are other add-ons providing instant-messaging capability to Firefox, but Yoono gets the special mention here because it supports the most chat and social-networking services: AIM, Facebook, Flickr, FriendFeed, Google Talk, Imeem, LinkedIn, MSN, MySpace, Twitter, and Yahoo Messenger.
Yoono also throws in widgets for Web note-taking (such as you can clip and save text or images you find on the Web); an RSS reader; e-mail alerting for your Yahoo Mail, Gmail and Hotmail accounts; and it can stream music from Imeem or Last.fm.
Yoono installs as a sidebar set to the left of the main Firefox window. When you click a button, it slides open as a wider control panel, but, otherwise, takes up minimal space.