I got Bad Request — Something went wrong. Please close this tab and try again

[bpidperygora]

Sync Login Error: “Bad Request — Something went wrong. Please close this tab and try again”

For several months, I’ve been unable to sign in to Mozilla Sync from Zen Browser.
Every attempt ends with the following error:

Bad Request
Something went wrong. Please close this tab and try again.

I’ve already tried all the common troubleshooting steps:

• Cleared all browser data, cookies, and cache (including for mozilla.org and accounts.firefox.com)
• Removed any stored profile or sync data
• Disabled all extensions
• Reinstalled Zen Browser
• Attempted login from a clean environment

Despite this, the issue persists every time.

Screenshot:

Additional details:

• This has been happening for several months across multiple versions of Zen.
• My Mozilla account works normally in regular Firefox.
• OS: Windows 11
• Zen version: 1.16.4b (Firefox 143.0.4) (64-bit)

Please help investigate or advise if there’s any way to reset or debug the sync authentication flow.