Websites actually can't detect the difference between regular Firefox and Firefox ESR, so there may be some misinformation in the instructions you are trying to follow. Can you provide a link to them?

Firefox 17 is rather old and vulnerable now.

The ESR versions are currently based on Firefox 45.0 and previous 38.0 which will end after 38.8.0 ESR update.

If you must use the old Firefox 17 for one outdated website then there is the portable Firefox option as it will be self contained and not touch your normal Firefox install or Profile with it. Here is portable Firefox 17.0.9 ESR you can use for this one site. You will have to copy+paste the whole url in Location bar.

The en-US locale is third one from bottom of list. http://sourceforge.net/projects/portableapps/files/Mozilla Firefox%2C Portable Ed./Mozilla Firefox ESR%2C Portable Edition 17.0.9/

Linking this way since http://portableapps.com/apps/internet/firefox_portable only has the current versions listed. It is safe as this project has been around since Firefox 0.8 without concerns.