Send to Kindle for Firefox is an extension by Amazon that allows you to send Web content to your Kindle. Previously only available for Chrome, it's a nice way to queue up long Web articles for offline reading. It also formats the content for easier reading, sans ads, menus, and other distractions. Here's how to use Send to Kindle for Firefox.
After you've installed Send to Kindle for Firefox, the Kindle button will appear in the upper right-hand corner of the Firefox toolbar. Left-click on it to bring up the options box.
The "Send to Kindle" option sends the content immediately to your Kindle. The first time you use it, you'll be prompted to log in to your Amazon account and choose delivery options, including which Kindle device(s) you want to send it to. You can also choose "Preview & Send" if you'd like to look at the layout first. If you want to only send selected text, choose "Send Selected Text."
That's it. If you want to change which devices you want your articles sent to, go to Settings; you'll be able to change delivery settings, as well as log yourself out of all active Send to Kindle browser extension sessions.