Can you create a new profile in addition to the existing one and report back if you can download anything with that one? Reinstalling Firefox almost never fixes issues, since any responsible settings reside in a profile that will be reused automatically.

Also (and perhaps first) try choosing No proxy in Firefox's connection settings if anything else is currently selected to see if that helps.

http://kb.mozillazine.org/Firefox_hangs#Hang_downloading_files

http://kb.mozillazine.org/Issues_with_Firefox#Downloading_opening_or_saving_files

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Unfortunately, none of these help me. I can download files with chrome just fine, but in firefox, every download fails no matter what I do. I tried installing an older version (from 54 to 52), with no luck.

My OS is win 10, my antivirus software is Panda.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a download manager.

This download manager works separately from the browser. The browser tells it what to download, and off it goes.

http://www.freedownloadmanager.org It is a powerful modern download accelerator and organizer for Windows and Mac.

As I've said in the original post. I can't download any add-ons either.

Sorry to ask but are you absolutely sure you created a new profile and started Firefox using that profile? In case you properly did, you should be able to select a profile at every launch of Firefox as long as no other copy is running, as shown in the support article. If you choose the newly created one, there should be no familiar bookmarks, settings etc. in Firefox.

Yes, that's exactly what I did.

I pressed win + R, typed in Firefox -P, created a new profile and opened Firefox again. I got Firefox with no bookmarks and default settings which was unable to download any file or add-on.

I can install themes, for some reason.

You could open up the Web Console in Tools > Web Developer (or press Ctrl+Shift+K), then select Network, visit a page with a download link, click the trash can on the left of the Web Console to clear the content and refresh the page. Then click the download link and see what happens. Note the statuses as well as padlocks and other entries that may look weird, and inspect HTTPS URLs in the Headers section on the right. If all this seems fuzzy, you could create a screenshot at that point and post it here. You can increase the size of the Web Console pane to give a better overview.

Personally I also suspect the Panda software to be interfering with your connection, especially since the issue concerns download requests while regular browsing is still possible. Could there be a Download Protection feature involved (I think so), i.e. have you checked for such options or recently changed any? I also read about versions that weren’t compatible with Windows 10, so that would require a special version. Or in other words, did downloading work before when Panda was installed and did the issue start recently, possibly after a Windows 10 update? Either way, I would probably uninstall Panda software completely, reboot the system and see if that helps. You could install it again afterwards, of course.

I've opened dev tools and the download request returned 200, so there's no problem there.

I think there was a panda update, and windows updates almost daily. I will remove panda and see how it behaves. But it's kind of weird because download works on other browsers I have.

Panda was interfering somehow. Disabling it didn't do the trick, but removing it did.

Thanks for the help.