I have an unusual Windows installation that routinely seems to trigger 8bit / wide char problems in software. It seems that FF57 (Quantum) is affected as well. The user-visible symptom is that starting Firefox fails with the well-known "Your Firefox profile cannot be loaded" dialog. Running firefox with "-p" or any other command line flag yields the same result. Since this indicated that I'm running into a problem I encounter from time to time let me give you some context: I'm running on Windows 10 which was originally installed in the Japanese-language version. My username contains the Umlaut "ü," and so does the path of my home directory. I later -- after creating the user account -- changed the UI language to English (some Japanese remains here and there). Japanese is a bit particular about paths (for historical reasons it uses or rather prints the Yen symbol as path separator, not the backslash). In fact some of the file handling stuff seems to happen in Codepage 932 which mixes backslash and the yen symbol. So correctness with the added Umlaut (which doesn't exist in Japanese) makes for a nice stress test of file name operations, and I have repeatedly run into very weird software failure. Back to my analysis: given my expectation that the Umlaut would be to blame I uses the standard Process Monitor development tool you can download from MS and looked out for calls that contain profile in the path. And lo and behold there is a suspect in the attached screenshot. Please observe how the umlaut between the two red boxes suddenly disappears, and the PATH NOT FOUND error appears. I searched the registry for possibly related entries, but didn't find any. I cannot add troubleshooting information because I cannot run firefox, as you might have already figured out.