Why would you want to run two different instances of Firefox? If you are a web developer or designer you must test your work across all the browsers you’d expect your clients or visitors to use. Using an iterative design process, it’s easy to apply tweaks and see the results across the typical browsers – Internet Explorer, Chrome, Firefox, Safari. But what if you needed to test your work out different versions of the browser, Firefox in this example. Unfortunately, in it’s default state Firefox really doesn’t allow for different versions to be installed, let alone running concurrently.

At this point you are probably turning to Firefox Portable – but this only solves the version portion of the problem. Even though Firefox Portable is self-contained and requires no installation, and therefor an assumption it does not  conflict with the installed Firefox – you still can’t execute both versions at the same time. The Firefox executable will first detect if there are other Firefox processes running irregardless of the version of the process. Quite a pain – but the fix is pretty simple!

Within Windows Explorer, navigate to the folder that contains your Firefox Portable “install”. Locate the file named FirefoxPortable.ini. Chances are, your copy of the file is actually located under the Other/Source child folder.

Copy this file and place it in the same directory as the FirefoxPortable.exe file.

Edit the FirefoxPortable.ini file by double-clicking on it – it should launch the Notepad application by default. Locate the preference item titled AllowMultipleInstances. The default value is false, simply flip it to true and save the file.

That’s it! The single line change will disable Firefox Portable from checking for already executing instances.