Find-and-Replace is an invaluable feature that most text-editors provide. Unfortunately while using Firefox for editing tasks, there is no such native alternative. But now there is FoxReplace, a Firefox extension, which adds find-and-replace functionality not just to text boxes and text areas, but also to entire web pages.
The simplest way to use this extension is to press Shift+F2, and enter the search string and replace string.
There are other advanced features like substitution lists, auto replace on page load and replacing URLs or images. Thanks Lifehacker