The same problem remains in Sisyphus:

firefox-3.5.3-alt0.20090831
$ rpm -qa '*xul*'
xulrunner-1.9.1.1-alt1.20090831
xulrunner-libs-1.9.1.1-alt1.20090831
xulrunner-gnome-support-1.9.1.1-alt1.20090831
$ 

+++ This bug was initially created as a clone of Bug #21657 +++

firefox-3.0.9-alt1.M50.1
$ rpm -qa '*xul*'
xulrunner-libs-1.9.0.9-alt1.M50.1
xulrunner-1.9.0.9-alt1.M50.1
xulrunner-gnome-support-1.9.0.9-alt1.M50.1
$ 

How to reproduce:

[ivan@dell ~]$ locale
LANG=ru_RU.KOI8-R
LC_CTYPE="ru_RU.KOI8-R"
LC_NUMERIC="ru_RU.KOI8-R"
LC_TIME="ru_RU.KOI8-R"
LC_COLLATE="ru_RU.KOI8-R"
LC_MONETARY="ru_RU.KOI8-R"
LC_MESSAGES=POSIX
LC_PAPER="ru_RU.KOI8-R"
LC_NAME="ru_RU.KOI8-R"
LC_ADDRESS="ru_RU.KOI8-R"
LC_TELEPHONE="ru_RU.KOI8-R"
LC_MEASUREMENT="ru_RU.KOI8-R"
LC_IDENTIFICATION="ru_RU.KOI8-R"
LC_ALL=
[ivan@dell ~]$ ls /home/ivan/data/OUT/bugreports/ALT/browsers/Russian-filenames
logo.doc  лого.doc
[ivan@dell ~]$ 

Now, start firefox an open this directory, click on the file with the Russian name (лого.doc), choose open in the menu.

Result:

openoffice is started, but it opens a new (empty) document. This wouldn't happen if one clicked on the file with the name in Latin letters (logo.doc). Also, if you typed 

$ oowriter /home/ivan/data/OUT/bugreports/ALT/browsers/Russian-filenames/лого.doc

it would correctly open the file. So the problem is with firefox not passing the arguments correctly to external programs.

Expected:

The filename is correctly passed to openoffice and the file is opened. This happens so in seamonkey-1.1.16-alt1 : seamonkey correctly opens this file in openoffice.