Can you not spoof Firefox 17 on that website that thinks it needs FF 17? User Agent Switcher works quite well for spoofing user-agents. Note that you also need User-Agent JS Fixer to be used with it.

If you do not like automatic updates, you stop them by setting these things in the about:config window:

app.update.auto = false
app.update.enabled = false
app.update.silent = false

If spoofing doesn't work, rather than turning off updates, use the ESR version, http://www.mozilla.org/en-US/firefox/organizations/

And does the website not work or does it just say it won't work? Two different things

You can install a portable (ESR) Firefox version to access websites that do not work with the current Firefox release.

The portable version comes with its own profile folder and doesn't interfere with the currently installed release version.

• http://portableapps.com/apps/internet/firefox_portable
• http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./