There have been previous TechBits articles that mentioned the IE View extension to Firefox. IE View is handy if you need to compare the view of a page in FF to the same view in IE, and it's essential if you use some site that (even well into the 21st century) still requires its users to have Internet Explorer. If you rely on such a site, you can configure IE View to always launch IE for links to that funky application.
But if you're a web site developer or tester, IE View may not be all that you want. You may also want to compare the view of a web page in Chrome, Opera, and Safari (or even Amaya, Epiphany, Konqueror, Maxthon, Lobo and...). In short, you may want the Open With extension for Firefox.
After you have installed Open With, your View menu should get populated with an "Open With <browser name>" item for each other browser that is detected on your PC. A similar menu item can optionally appear in several other contexts as well. To set up Open With for various contexts, pull down the FF Tools menu and choose Add-ons, then select Extensions and click the Options button for the Open With extension. A new tab will open showing the Open With settings that you can tune for the View menu, context menu, tabs and the tool bar.
What about different versions of the same browser? You want to test FF 3.6 and FF 7, and IE 8 and IE 9, right? Sadly, this is often not possible without multiple PCs. Even in cases where it is possible to have two versions of the same browser on one machine, it tends to get a bit funky to manage those installations. Depending on your OS license and hardware capacity, you may benefit from running "another PC" inside a VirtualBox or another virtualization platform, but that level of complexity is far beyond what I can cover in a short blog posting.