In Firefox, there's an accessibility option which is quite useful, which is "Search for text when I start typing". This feature is very effective if you want to search strings in a long file, for example you would like to find word related to alsa in Linux's kernel changelog. All you have to do is turn this feature on and wait until the changelog is fully downloaded and start typing for alsa. A search box will appear in the bottom of the application.
A problem can occurred when this feature is turned on. When the page has a form and it hasn't been finished rendering and we type something in the text box to fill our user name for example, sometimes Firefox thinks that we're searching for text and not filling in the forms. Even after the page has been finished rendering, we couldn't fill in the form also. The solution is to turn the feature off again or you will have to refresh the page and wait until it's finished rendering and downloading.