What OS? What version of Firefox? Did you try with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Firefox version 134 and Windows 11. Literally just installed both within the last hour or two. Troubleshooting mode just asked me to refresh the install which did nothing.

I also tried downloading an older version to see if that would help but the issue remained. So I uninstalled everything and reinstalled version 134.

What security software are you running?

How about using windows snip or sharex tool to record your screen and post on youtube. If Windows Pro version, try testing in Windows Sandbox (VM) to see if you have the same issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

As this is a fresh install of Windows 11, I've not installed anything beyond that. As soon as the install finished, I opened Edge so I could download Firefox and I've been working on that issue ever since. There's nothing on this system but stock Windows, nothing else has been installed beyond JavaScript to see if that would help.

However, this yesterday this was a Windows 10 system, also from a fresh clean install. I did the same thing there that I did today. Opened Edge, installed Firefox, unable to log into Sync. After fiddling for a while, I let it go and today I decided to try upgrading to 11 to see if that would solve the issue. No such luck.

Here's a video that shows Windows Sandbox mode. The same issue happens exactly as it does in non-Sandbox mode.

https://youtu.be/1fiCsSLUvMM

Just to confirm, it's the same issue in both Windows and Windows Sandbox? It shows waiting to connect to accounts.firefox.com in the bottom left corner of the browser . see screenshot What is your network setup? WiFi or plugged in? Who are you using for DNS on your computer, in the browser, and your router?

Ping and traceroute It all point to google.  ;-)) Let me log out and back in to see what happens.

I was able to log out and back in with no issue What are your security settings? see screenshot

Yes, I confirm it's exactly the same in Windows and Windows Sandbox. They are exactly identical to each other.

My computer is plugged directly into my Netgear Nighthawk XR500 router. My DNS comes from my ISP (Metronet). The DNS address in my router is 206.225.75.225 and 206.225.75.226. DSNleaktest.com shows DNS 108.162.217.153 and 108.162.217.154 in the browser.

Here are screenshots from the router and the browser.

I would suggest you get all involved(computer, browser, and router) on 1 dns service. Use this tool to see which one is best for your location, I'm in NJ. https://www.grc.com/dns/benchmark.htm see screenshot because you have to scroll down a bit to get to the download button. Also see what I get for hops to the different servers. see screenshot See your dns compared to cloudflare using traceroute.