Gnash has moved from bzr to git, at least for the moment - Savannah's bzr setup is slow and unlikely to improve, so the choice is git or Launchpad, apparently. (Thinking selfishly, I'm a lot more familiar with git than with bzr, so I hope it stays this way.) My instructions from July have changed only slightly:
git clone git://git.sv.gnu.org/git/gnash
cd gnash
./autogen.sh
./configure --enable-renderer=agg \
--enable-media=GST \
--enable-gui=gtk \
--with-plugins-install=system
make
sudo make install
sudo make install-plugins
Buried near the end of the bug reports wiki page is a useful tip for debugging - export GNASH_OPTIONS=writelauncher before starting the browser, and gnash will write out the instructions for starting a local copy.
In practise, these instructions seem to appear as a file called /tmp/gnash-debug-1.sh, and that will reference a file matching /tmp/gnash-cookies.* - so don't bother trying to dig through the debug log.
With this tip, it's possible to get the browser out of the loop, and focus on debugging gnash (which should be enough for most problems). At least, that's what I hope - I've spent the last hour or so just figuring this stuff out. Hmpf.