Due to a bug ("feature") in the PDF plugin module for
Adobe Acrobat Reader, use of the plugin from within a
browser or email reader results in the creation of the
file "C:\nppdf32Log\debuglog.txt" in whatever working
directory the plugin runs in. In Thunderbird, this
creates a new bogus mail folder. Various posters have
noted that creating a symbolic link with this name
pointing at "/dev/null" will prevent the data from
accumulating.

However, the file will still exist. An alternative is to
do a binary patch to fix the problem. The filename in
question is a string constant in the module "nppdf.so",
of which your system may have multiple copies associated
with different browsers, email handlers, etc. In each of
these use 'ht' to search for "C:\nppdf32Log\debuglog.txt"
and change it. In my system (Slackware 14 with Adobe
Acrobat Reader 9.5.1 installed), nppdf.so is 179552 bytes
long and the offending filename is found at location 0x02232b.
Change this to ->2f 64 65 76 2f 6e 75 6c 6c 00 (hex)
/ d e v / n u l l \000 (ascii)

Now, no file is created and no annoying "mail folder" appears.

Source for ht is available at http://hte.sourceforge.net or
via the SlackBuild at www.slackbuilds.org. I used ht-2.0.20,
since the slackbuild references 2.0.18 which is 3 years old,
and 2.0.19 has ELF bugs. "It just works"