The IRS FIRE testing site is here

https://fire.test.irs.gov

The testing site looks identical to the production site. But testing requires a unique User ID, Password, PIN and secret phrase.

You can submit as many test files as you like. You can test original data or upload test files of 1-step or 2-step corrections. A test file will have a "T" in Field Position 28 of the transmitter record. The IRS will email you the results from testing in 1-5 days or you can log back into the testing site and check the file status.

Combined/Federal State Filing Program (CF/SF)
You are required to submit a test file to become a member of the CF/SF program. You need to put together a file with one payer and at least 8 recipients and upload the file identified as a CF/SF file. If the file status is Good or Accepted, call the IRS and ask them to elevate your TCC number to include CF/SF. It typically takes the IRS 1-2 days to elevate your TCC number to include CF/SF and you only need to successfully test and elevate your TCC one time.