Writing efficient user interfaces is the main maxim, here at Vimperator labs. We often follow the Vim way of doing things, but extend its principles when necessary.
Towards this end, we've created the liberator library for Mozilla based applications, to encapsulate as many of these generic principles as possible, and liberate developers from the tedium of reinventing the wheel.

Currently, these applications are using this library:

Vimperator

Vimperator, the flagship project from Vimperator labs, creates a Vim-like Firefox. 

Vimperator is a Firefox browser extension with strong inspiration from the Vim text editor, with a mind towards faster and more efficient browsing. It has similar key bindings and you could call it a modal web browser, as key bindings differ according to which mode you are in. For example, it has a special Hint mode, where you can follow links easily with the keyboard only. Also most functionality is available as commands, typing :back will go back within the current page history, just like hitting the back button in the toolbar.

Muttator

Muttator is to Thunderbird what Vimperator is to Firefox. Combines the best aspects of Vim and Mutt.

Muttator is a free add-on for the Thunderbird mail client, which makes it look and behave like the Vim text editor. It has similar key bindings and you could call it a modal mail client, as key bindings differ according to which mode you are in. For example, it the same keys can select the next message while the message list has focus or can scroll down an existing message when the message preview is selected. It also adds commands for accessing most Thunderbird functionality. E.g., :contact -lastname "Vimperator" vimperator@mozdev.org will add Vimperator's mailing list to your address book.

Get involved

For now, we have just focused on these three Mozilla applications because of time constraints. If you want other host applications like Instantbird or Sunbird to make use of our liberator library, feel free to contact us.