Open With is a time-saving Firefox extension that will quickly open the current web page in another of your installed browsers.

So if you're a web developer, say, and want to see what your site looks like in IE, there's no need to manually launch the browser and enter a URL. Just click View > Open with Internet Explorer (in Firefox), and an IE window will open to display the current page.

If the extension doesn't detect your installed browsers (or maybe you're using a portable build), it's easy to add additional entries.

Version 6.6 (Changelog):
    - This version fixes a bunch of bugs and fixes a long-standing problem - communicating with a second instance of Firefox. If you have Open With 6.6 installed in multiple running profiles, they will now talk to each other instead of trying to start a new operating system process, so you should no longer see the "Firefox is already running" message.

Verdict

If you regularly have to view the same page in multiple browsers then Open With could save you a great deal of time