Forgot to mention this affects Ethernet and Wifi, however Wifi with Firefox open gets slightly better speeds than Ethernet. Ethernet is much faster than Wifi with Firefox closed.

My best guess would be some sketchy add-on to firefox, p2p activity in background and such, but you said you did reset firefox so there should be no add-ons.

This looks like network driver sets limits on your network, prioritizing Firefox over other apps, setting download limit to slow value if firefox app is active. I think killer ethernet had some stuff like this: https://www.intel.com/content/www/us/en/download/19779/intel-killer-performance-suite.html

Maybe try installing clean driver only without any additional "control software" if it is present. Or maybe some other software on your pc has these limits, for example anti-virus or some download tool?

Unfortunately I have no more ideas.

Thanks for the response and suggestions. I appreciate it.

Even if Firefox is the only thing open, anti-virus/firewall is deactivated, Firefox itself has a slow download speed. If Chrome is open to test the speed, if I close Firefox while it's being tested the d/l rate will increase immediately.

There is no special Ethernet software such as Killer Performance Suite, no p2p in the background, and Firefox was a fresh no add-ons install. I didn't install any control software, and it only seems to be Firefox affecting/affected. It's a Realtek Ethernet driver I've tried a few times to cleanly install.

A frustrating mystery.

Well it's the silliest answer. Changing the network driver to an older version fixes this issue. I never would have thought it to be that, based on it only affecting the speed when Firefox is open. 'Updating' the driver from 'device manager' with some trial and error has resolved the issue.

I have the same problem with even more dramatic results. And changing drivers and USB-C 2.5 GbE adapters did not fix the problem for me. My speeds go from 2200 Mbps to 750 Mbps or lower whenever I run Firefox. The speeds usually return to 2200 Mbps as soon as I stop using Firefox AND stop it from running in the background. No other internet browser I've tried creates this problem. I've confirmed this problem with several Comcast tech support people as well, in order to verify Comcast was not the problem. It is not. I've tried numerous alleged remedies & none work other than to stop using Firefox. Enough people have been complaining about this problem that Mozilla support can no longer hide from their problem. The Mozilla developers have to know of this problem. And it's about time they admit to it & fix the problem.