That could be about a new security feature that blocks unsafe downloads, but can interfere with downloading files. You can set dom.block_download_insecure => false in about:config to disable this feature.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Hey Jeff! It looks like a new security feature was released with Firefox 125 that blocks downloads from non-HTTPS urls. The devs found out it causes the wrong file or a corrupted file to be downloaded on certain sites that implement downloads in a different way. They posted a note in the "Unresolved" section of the latest release notes: https://www.mozilla.org/en-US/firefox/125.0.1/releasenotes

Note from Release Notes: The recently-shipped functionality more proactively blocking downloads from untrusted sources is not working as intended and causing impaired ability to download files in legitimate situations. We are working to address this ASAP via disabling the feature remotely for existing installs and will also revert the change for the upcoming Firefox 125.0.2 release.

To verify if this the same issue you're seeing, you can go to about:config & search for dom.block_download_insecure & double-click to change it to False. Then close Firefox, reopen it & try downloading those PDF files again.

Please report back if that fixes this problem.

about:config > dom.block_download_insecure - false Does resolve the issue with the corrupted Downloads

I cannot download PDF files with Firefox (126.2 now) on my iPhone, or all I get is corrupted files.

About:config does not seem to be available on iOS :(