Which version? For me 90.0 shows the "Please wait" message which some JavaScript or something is supposed to replace, and 93.0 force downloads it, even when I try to open it from a local file.
From the release notes: "Firefox PDF viewer now supports filling more forms (XFA-based forms, used by multiple governments and banks)."
https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-...