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.
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[Closed] Firefox Portable 93.0 "Another instance of Firefox is already running."
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.
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.
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.