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.
In theory Open With should detect your browsers at installation, adding a link for each to the Firefox "View" menu, so you've no need to do anything else.
In practice, though, we found it didn't detect our copy of Chrome. And of course you may install a browser later, and want to add it to the list. Fortunately you can also manually add a browser (or any other program) to Open With's items. Just click Tools > Add-ons, choose Open With, click Options > Manual Entries and customise its selection to suit your needs.
And if you'd prefer not to have your Open With items on the View menu, check the Options dialog display tab for alternatives, including placing them in a submenu, the main context menu, the tab bar, and more.