With the AllowMultipleInstances=true in FirefoxPortable.ini (that's in my C:\FirefoxPortable folder along with the FirefoxPortable.exe file) when I try to open a 2nd instance it gives the same error popup dialog again that I also got a few months ago when I was still on a (slightly) earlier version of Firefox Portable, and that used to be fixed in an updated version until it came back as of 93.0.
I just installed two copies and it works without issue here in both, whichever one I start first. Is your INI file properly ANSI or UCS2-LE? If it's UTF8, it won't work. The error you are showing means that the launcher itself is generating it which means it isn't seeing that AllowMultipleInstances setting.
Sometimes, the impossible can become possible, if you're awesome!
The FirefoxPortable.ini I'm using is UTF8. If I edit it in Notepad which uses Western 1252, opening FF it goes crazy and then, a different one every time, loads some random page, sometimes a "not found." Then opening a second instance of FF presents only the splash screen momentarily and in about 30-45 seconds the FF crash reporter dialogue.
I had to troubleshoot this for quite some time a while back and used default profiles to make sure my profile wasn't the issue. And I don't recall what source I found that clued me in on ini files character encoding weirdness.
Cheers.
I know that Notepad++ is set by default to "helpfully" switch ANSI files to UTF-8, which breaks Windows INI files.
Sometimes, the impossible can become possible, if you're awesome!
Thank you for fast reply. UTF-8 or ANSI, didn't make a difference (using "Save As" option in notepad on Windows 11). Deleting the Mozilla folder from AppData, LocalAppData and ProgramData did the trick. But I also have another problem so I will make a new thread.