Save button... You mean download right? Steps and link to replicate your issue would help. If you go to this site https://www.irs.gov/downloads/irs-pdf and click any pdf, do you have the option to save or download?

Judging from the Find button toward the left end of the bar, it is a customized viewer provided by the site and not Firefox's built-in PDF viewer.

Sites usually adopt a custom viewer when they want more control over what you do with their content -- this way Firefox never receives the PDF directly.

If you click the overflow menu button on the right side (>>), do they offer any options there?