Folders are used on Windows and MS-DOS filesystems. The proper name for file containers on all Linux filesystems is directory. To improve quality of Thunar, I would suggest that the sources be modified to reflect this terminology. Using grep and sed, I was easily able to replaces instances of the words folder with directory, so I can't imagine that it would be difficult.
Err, a folder is a general concept, that is independent of a special file system. Such special files were called folders long before FAT was invented. I think you are really confusing things here.
That may be, but modern terminology has evolved such that references to filesystem containers has split with platforms. The general consensus among users is that UNIX-like operating systems refer to said containers as directories because of the hierarchical nature of the tree, much like other uses of the term 'directory', whereas Windows uses the term folders as a reference to the nature of its filesystem to reflect that of a paper filing system. This is because of the drive-letter vs root directory structures of the operating systems. I do however, stand corrected in my reference to filesystems. While one can mount a FAT or NTFS filesystem within a UNIX directory, its containers would not be considered folders in this case, but directories due to the platform on which they are used.
Err, where are those users? We use the same terminology as GTK+ and other GTK+-based software, to be consistent with other apps. Maybe the users you asked were using KDE before where IIRC the term directory is used instead, but KDE is not Unix. I'm also missing the whole point in your argumentation. A folder is a folder from the user point of view. Whether the syscall is named mkdir or mkfol is totally irrelevant. If windows happens to use the same terminology, that's even better, so people don't need to get used to new terminology. BTW: Sitting in front of an OS X box right now, checking Finder's terminology, it also uses the term folder, still its based on Unix. Again, I think you are really confusing things here.
Your response got me thinking, so I asked my UNIX professors for their input on this issue. They, having learned to use the computer with UNIX, before DOS and Windows were around, refer to file containers as directories regardless of platform. After a discussion, I think the conclusion to which we came was that the term directory and folder seem to be interchangeable and depend upon the interface the user is given, with folder becoming a prominent term with the advent of the GUI and these containers were represented with icons depicting true paper folders. So, I suppose this bug really is invalid, and being the developer, it is your prerogative to call them whatever you wish. I am not sure what it is, but something about the term folder just irks me, so I will continue to sed the sources and rename them myself. That is the glory of open source software, I suppose. I do apologize for your time I have wasted. You are doing an excellent job with the development of XFCE, and I am sorry for taking what little time I am sure you have away from that. Please, keep up the good work; I enjoy using your software. You have my word I will not submit any more frivolous bugs :D