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On Linux - tested on: Arch linux @ Debian Squeeze, this fails
 python -c "__import__('webbrowser').open('http://python.org')"
The exception thats raised is:
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "/usr/lib/python3.2/webbrowser.py", line 70, in open_new_tab
 return open(url, 2)
 File "/usr/lib/python3.2/webbrowser.py", line 62, in open
 if browser.open(url, new, autoraise):
 File "/usr/lib/python3.2/webbrowser.py", line 276, in open
 success = self._invoke(args, True, autoraise)
 File "/usr/lib/python3.2/webbrowser.py", line 239, in _invoke
 stderr=inout, preexec_fn=setsid)
 File "/usr/lib/python3.2/subprocess.py", line 736, in __init__
 restore_signals, start_new_session)
 File "/usr/lib/python3.2/subprocess.py", line 1330, in _execute_child
 raise child_exception_type(errno_num, err_msg)
OSError: [Errno 9] Bad file descriptor
Noticed that this wont raise an error when stdin arg isn't passed to Popen:
line 237:
 p = subprocess.Popen(cmdline, close_fds=True, stdin=inout,
Removing tdin=inout makes firefox load ok, however this is there for a reason so its not a useful fix ofcourse.

The problem lies here:
/* Close pipe fds. Make sure we don't close the same fd more than */
/* once, or standard fds. */
if (p2cread > 2) {
 POSIX_CALL(close(p2cread));
}
(c2pwrite > 2) {
 POSIX_CALL(close(c2pwrite));
}
if (errwrite != c2pwrite && errwrite > 2) {
 POSIX_CALL(close(errwrite));
}
If p2cread == c2pwrite (which is the case here since /dev/null is passed as stdin and stderr), we end up closing the same FD twice, hence the EBADF.
Just passing 
-(c2pwrite > 2) {
+(c2pwrite > 2 && c2pwrite != p2cread) {
 POSIX_CALL(close(c2pwrite));
}
Solves this (but you probably also want to check for (errwrite != p2cread) when closing c2pwrite).
Note that the Python implementation uses a set to avoid closing the same FD twice:
# Close pipe fds. Make sure we don't close the
# same fd more than once, or standard fds.
closed = set()
for fd in [p2cread, c2pwrite, errwrite]:
 if fd > 2 and fd not in closed:
 os.close(fd)
 closed.add(fd)
It might be cleaner to use a fd_set, i.e.:
fd_set set;
FD_ZERO(&set);
FD_SET(0, &set);
FD_SET(1, &set);
FD_SET(2, &set);
if (!FD_ISSET(p2cread, &set)) {
 POSIX_CALL(close(p2cread));
 FD_SET(p2cread, &fd);
}
if (!FD_ISSET(c2pwrite, &set)) {
 POSIX_CALL(close(c2pwrite));
 FD_SET(c2pwrite, &fd);
}
if (!FD_ISSET(errwrite, &set)) {
 POSIX_CALL(close(errwrite));
 FD_SET(errwrite, &fd);
}
But maybe it's just too much (and also, fd_set can be defined in different header files, and while I'm sure it's async-safe on Linux, I don't know if it's required as part of a standard...).

Attached is a patch checking that no FD is closed more once when
closing pipe FDs, along with an update for test_subprocess.

That probably won't make a difference here, but be aware that we switched to Mercurial and the SVN repo won't get updates anymore.
See http://docs.python.org/devguide/ and http://docs.python.org/devguide/setup.html#getting-the-source-code for details.

Is this a webbrowser or subprocess bug? Please add the corresponding expert from http://docs.python.org/devguide/experts to the nosy list.

subprocess in 3.2 bug from the looks of it. not sure if 2.7 or 3.1 are impacted at all, i'll remove them from the list after confirming.

New changeset 08daf3ef6509 by Gregory P. Smith in branch 'default':
Add unittests demonstrating issue #11432.
http://hg.python.org/cpython/rev/08daf3ef6509
New changeset adcf03b074b7 by Gregory P. Smith in branch 'default':
Fix issue #11432. if the stdin pipe is the same file descriptor as either stdout or stderr
http://hg.python.org/cpython/rev/adcf03b074b7
New changeset ead52adcd0c8 by Gregory P. Smith in branch '3.2':
Fix issue #11432. if the stdin pipe is the same file descriptor as either stdout or stderr
http://hg.python.org/cpython/rev/ead52adcd0c8
New changeset ad2bd8d338b0 by Gregory P. Smith in branch '3.2':
Add unittests demonstrating issue #11432.
http://hg.python.org/cpython/rev/ad2bd8d338b0 

New changeset 82e010943c68 by Gregory P. Smith in branch '3.2':
issue 11432 news entry.
http://hg.python.org/cpython/rev/82e010943c68
New changeset 8d3bcf57977b by Gregory P. Smith in branch 'default':
Misc/NEWS entry for issue 11432
http://hg.python.org/cpython/rev/8d3bcf57977b 

With fix, test, and news in 3.2 and 3.3, is anything left to do?