Having multiple versions of XULRunner installed on one machine is simpler than you might imagine. If for whatever reason, you want to run Conkeror with a particular XULRunner version, you can be up and running in minutes.

For XULRunner versions 5 and above

1. Install your desired version of XULRunner and simply issue the command:

    [xulrunner-program] [/path/to/conkeror/application.ini]

   For example, on Debian with Conkeror version Debian-0.9.3+git110719-1 and xulrunner-7.0a2 installed from the Mozilla repositories at http://mozilla.debian.net/, the following command line was successful:

    xulrunner-7.0 /usr/share/conkeror/application.ini

2. You can then make sure that your Conkeror is using the desired version of XULRunner by checking the value of M-x version in Conkeror.

3. Optionally, create a convenience shell script for yourself, such as ~/bin/conkeror7, to start conkeror with the desired XULRunner version.

For older versions of XULRunner

1. download a binary archive of the XULRunner version you want, for your platform.
2. extract it to a directory somewhere.
3. enter that directory in a terminal.
4. issue the command:

   ./xulrunner --register-user

5. there will now be an entry in the directory /etc/gre.d/ for the new XULRunner version.

6. change to your conkeror directory. It is preferable to use Conkeror from git for this part, as not all OS-specific packages will come with the requisite scripts.
7. issue the following command to ensure that Conkeror will be able to find the version of XULRunner that you just registered.

   ./contrib/list-xulrunner-versions

8. take note of the exact version string of the XULRunner version you want to use. You will use this string in place of the ### marker in the following command.

   CONKEROR_XULRUNNER_VERSION=### ./contrib/run-conkeror

9. if all goes well, Conkeror starts, and you can verify that it used the version of XULRunner which you intended by doing M-x version.