Hi all, I'm struggling with certain downloads and Firefox when using a web proxy. The proxy settings are done in Windows 10 and Firefox is set to use system proxy settings. Users are authenticated automatically at the proxy using kerberos (Windows AD environment) On the proxy (Squid) I can see the failing files downloaded and I can download these files being transferred to firefox via the proxy port when doing packet capture. Nearly all sites and services work. Only some are affected. When using other browsers (MS IE, MS Edge, Chrome) these downloads work perfectly. When disabling Proxy on Firefox these downloads work, too. Only with this combination Firefox + Proxy I get some failures. This is one example (that's a PDF datasheet): https://www.ergoswiss.com/de/services/cad-hydraulikhubsysteme/pumpen/pumpe-pf-3-4-anschluesse/?tx_downloads_downloadscategory%5Bdownload%5D=1753&tx_downloads_downloadscategory%5Baction%5D=download&tx_downloads_downloadscategory%5Bcontroller%5D=Download&cHash=9351869892df2bd6c6fbebbd2e74e862 And this is the error message Firefox gives me: C:\Users\[changed for privacy]\AppData\Local\Temp\64gqW0kX.pdf.part could not be saved, because the source file could not be read. Try again later, or contact the server administrator Again this works in Firefox without proxy and It works in the three other browsers with and without proxy. And only few files are affected. Besides checking logs and doing packet capture I'm running out of ideas. Where do I continue troubleshooting? Firefox 84.0.1 Windows 10 Pro 1909 x64