Firefox: If you frequently use sites that require browser-specific extensions, test websites or any other need compels you to open links in alternate browsers, Open With makes hopping from Firefox to another browser a one-click affair.
Once installed Open With adds links to the browsers available on your machine in the View menu, at the end of the tab bar and in the right-click context menu. Clicking on any of them will open your currently viewed page in the browser of your choice. There are no additional frills or settings, just simple and solid integration with the other browsers on your computer for one-click page opening.
Have a favourite tool for cross-browser functionality, Firefox-related or otherwise? Let's hear about it in the comments. Open With is free and works wherever Firefox does. Thanks dude004!