I have the same problem.

Running a download speed test, I get close to 50 Mbps with Chrome. With Firefox on my laptop I get around 20 Mbps on average. It varies a lot: very rarely I can get close to 50 Mbps, but it can be as low as 5 Mbps. This is not because of my Internet provider. I have tested this numerous times. When downloads are terrible with Firefox, I can get fast downloads with Chrome — always. I have never got below 47 Mbps with Chrome.

I have spent three days trying to solve the problem. Running Firefox in safe mode didn't help, nor did running Malwarebytes or doing a full scan with Total Antivirus.

Finally, I reset the operating system (Windows 10) and reinstalled Firefox as my first and only software installation (besides those loaded as part of Windows) — with no Firefox add ons. This didn't help either.

Curiously enough, the one thing that works is to run Windows (not Firefox) in safe mode. However, trying to replicate this in normal mode by disabling start up items or services didn't work. Specifically, I used the commands "net start > norm.txt" and "net start > safe.txt" (running cmd in Administrator mode) to find what services were running in both normal and safe mode and I then disabled using System Configuration all the services running in normal mode that were not running in safe mode. This didn't help.

Follow up: I was due for an upgrade to my SSD, so I did the upgrade, which involved a new installation of the operating system. The speed problem went away. I still have no idea what caused it.

I haven't reinstalled all of the programs. I will test the download speed as I do. If a particular program installation causes the problem to reappear, I will post back.