Hello, I understand that downloading a form from a particular site is not working. I am happy to help.

Please first make sure to clear the cache of the site to make sure there are no corruptions in the cache: How to clear the Firefox cache

Second please make sure to check out these troubleshooting steps: What to do if you can't download or save files

Also, you could temporarily turn off Firefox's pop-up blocker if you think that is the reason that the new window is not opening. See: Pop-up blocker settings, exceptions and troubleshooting (second section of the article, "Pop-up blocker settings").