Running multiple instances at the same time...

It can be a little complicated, but you can achieve it. You need to manually set a setting in the FirefoxPortable.ini. It's explained in the included readme.txt as referenced in the help.html file. Copy the FirefoxPortable.ini from Other\Source next to FirefoxPortable.exe and set AllowMultipleInstances to true.

There are multiple caveats with this. It will leave directories and files behind on every PC you run it on. These files should not contain any personal information and will usually be blank, but we make no guarantees. When running in this setup, you won't be able to launch files into Firefox by associating them (either in Windows or Portable File Associations or similar) as Firefox won't know which process to pass the file to.

Like the majority of applications, Firefox and other Mozilla-based apps are not designed to run multiple instances pointing to multiple sets of settings at the same time. Neither we nor Mozilla support or recommend this setup for anything except testing, so you're fully on your own using it.

I may be the only person on planet Earth doing this, but I am running two Windows 10 accounts simultaneously, one for work and one for pleasure. These are not separate windows, they are actual separate windows 10 accounts, each with its own settings, logs, windows, etc.I was able to successfully run Firefox portable in both independent Windows 10 accounts simultaneously until about a year ago or less. Then only one would run at a time even though I had windows make it impossible for one account to see any files in the other account. The good news is that I was able to finally solve the issue by following the procedure of moving the sample ini file up to the main directory and changing the multiple instance flag to be true. I did that for both portable Firefox apps in the separate Windows 10 accounts.

Since these are running on an island so to speak, I could not and still cannot figure out how one knows that the other one is running even though it cannot see any files in the other account. It must be in the OS somewhere or the kernel where it is logging that there is a Firefox running somewhere in the computer. Because the accounts in the two different Windows 10 accounts cannot see any data from the other, there is no chance of cross-contamination. It's hard to write to a directory that doesn't exist from your realm.

So for anyone else wanting to run Firefox portable on two different unconnected Windows 10 accounts separately simultaneously, make the simple change and I think you will be fine. You may want to do like I did and make the portable directories unreachable and unviewable from any account except the one actually running that instance of Firefox. Happily, the change in each Windows account took a couple of minutes and could've easily been backed out had it failed. I'm a bit concerned that if Firefox can see other versions of it running in other Windows accounts, is this a serious security flaw in Windows 10? I thought that each account in Windows 10 was supposed to be isolated from the other and not really know what's happening in other accounts. I suspect that this is a bug rather than a feature of Windows 10.

This is one of the most frustrating problems. I have a simple icon in the taskbar that is a shortcut to FirefoxPortable. If I click the icon twice, the second time says there can't be multiple instances of the app running. I'm not even sure how anyone can say that's not a supported use case, because it's pretty common to want to use more than one browser window.

If you launch FirefoxPortable, it ultimately calls firefox.exe inside of either the Firefox or Firefox64 folder. If you launch FirefoxPortable.exe and then right click the app that opens in the taskbar and select "Pin to taskbar", it creates a shortcut to firefox.exe instead of FirefoxPortable.exe.

Furthermore, when you launch Firefox from that pinned shortcut, it will not load your previous Firefox profile settings. If you try to go back and run FirefoxPortable, it will say that you can't run multiple instances of Firefox.

If you right click on the Firefox app in the taskbar and select "Firefox" to open a new window, it will either:
- load Firefox in a totally different profile
- or popup the multiple instances error