One of my most desired features in Firefox is to have a “Split Window” view, like you can have in Microsoft Word (and probably others too). It displays the same document in two panes, one above the other and lets you scroll to a different position in each each. Immensely useful if you’re trying to keep track of a previous part of a document while reading further on.

The application in web pages is arguably better still as you are able to scroll to the “Reply” box on a web forum, while scrolling up to some post you want to quote without losing your place.

While there’s no extension to do this, you can use a Bookmarklet (a piece of JavaScript, saved as a Bookmark) to apply the change.

split document — bookmark or click on this link

You can click on the above link to see a demonstration, or bookmark it to make it accessible from your Bookmarks menu on all/any page.

It works very well, though as a word of warning I’d advise against using this after you’ve started writing in a form on a website as it will reload the page, losing anything you’ve written. It’s a drawback that maybe a proper Firefox extension could evade.

Update:

After a little testing, this particular bookmarklet doesn’t work in IE or Opera so it’s probably Firefox users only (we have no testing Mac at work right now). There’s no reason why such a thing couldn’t be better coded to work on multiple browsers though, or at least there shouldn’t be.