Firefox problems with Unicode file names

[Sad Clouds <cryintothebluesky%googlemail.com@localhost>]

Hello, I'm running Firefox 47.0 on NetBSD-7 built from recent stable
pkgsrc. When filenames contain Unicode characters, Firefox is unable to
load those files from a local file system, i.e. it displays a message
"File not found"

I have my environment setup for utf-8

$ locale
LANG="en_GB.UTF-8"
LC_CTYPE="en_GB.UTF-8"
LC_COLLATE="C"
LC_TIME="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_ALL="en_GB.UTF-8"

I fact, my email client (Sylpheed) is able to open and read the same
files that Firefox fails on, so I'm inclined to think it may be Firefox
+ pkgsrc specific issue.

Anyone seen this before with NetBSD? Is it possible that pkgsrc patches
may have introduced this issue?

Here is how to reproduce it:

$ touch /tmp/ab\U+00E2\U+0080\U+0099c.htm
$ ls /tmp/ab*
/tmp/ab?c.htm

Firefox is not able to open the above file