Here is the image

%20 is the escape code for a space character, so you can see this %20 in case a file name contains one or more space. I'm not sure whether Firefox is supposed to show the %20 as part of the file name when you save a file.

How are these files send?

Is that via a content-disposition header or are you saving the files as a normal link (Save Link As)?

Just to be sure that an extension isn't interfering:

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thank you for your help, but it didn't work. The %20 are still there. I turned add-ons off. Save mode did't help... This is a file I downloaded witch Chrome: NS wil netwerk uitbreiden met regiolijnen.pdf This file I downloaded with Firefox: NS%20wil%20netwerk%20uitbreiden%20met%20regiolijnen.pdf

I can confirm this.

The Network Monitor shows:

Content-Type: application/pdf
Content-Disposition: attachment;filename=NS%20wil%20netwerk%20uitbreiden%20met%20regiolijnen.pdf

I don't how to prevent Firefox from showing the escaped file name.