"another instance of firefox is already running" problem

are you using AllowMultipleInstances in the .ini?
Also, make sure you have the up to date .ini because older ones are incompatible.
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R McCue
Cube Games
People who didn't need people needed people around to know that they were
the kind of people who didn't need people.
(Maskerade)

It's a simple fact. When the launcher starts up, it checks the running processes list for firefox.exe. If it finds it, it checks to see if it's the copy of Firefox contained within Firefox Portable that's running. If it is, it passes on it to start another one. If it isn't, it pops up that error message.

So, when would you see that message?
1. If firefox.exe is already running. It may be open. It may have partially crashed but the process is still running. (Check task manager)
2. You renamed FirefoxPortable.exe to Firefox.exe, in which case it is mistakenly detecting itself. This is why the instructions say that you can name it to anything you want except firefox.exe (useful if an admin has set it so Windows won't run FirefoxPortable.exe)

So why is it like this? Because Firefox itself is *DESIGNED* so that you can't start multiple copies with different profiles by default. Again, this is by design so problems don't arise. There is a special environment variable you can set that will allow Firefox to run alongside another copy of itself which is primarily for developers. The launcher exposes this to the end user by providing an INI option called AllowMultipleInstances. It's fully detailed in the readme.txt which is linked to right from the help.html file (you did read the help.html file before posting your rant, right?)

So why don't I use multiple instances by default? Because then the launcher wouldn't be able to keep track of whether its Firefox was still running or not and would leave behind files on the local PC (and not work in Live CD mode). Which isn't very portable is it?

The only way to run Firefox alongside another copy without using the special environment variable is to run it inside a virtual machine. Virtual machines require admin rights... so they're not truely portable.

As to why people use Firefox Portable? Most people don't care about running it alongside a local copy. The top two reasons to run Firefox Portable are (1) to be able to use Firefox on machines that don't have Firefox (like work or school) and (2) so you can take your bookmarks and extensions with you... not necessarily in that order. It should be obvious that most people don't use this option due to the fact that Firefox Portable has been downloaded 1.9 million times... and the concept of multiple instances has only been discussed a handful of times.

If you rename your Firefox Portable.exe file, you will get the message
"another instance of Firefox is already running".So don't rename your Portable Firefox executable file.

Check
here