First try these troubleshooting steps:

• Firefox Sync troubleshooting and tips
• Clear the cookies from accounts.firefox.com in about:permissions (put that into a new window and search, then click "Forget this Site")
• Do you have a firewall/antivirus that may be blocking this connection?

Can you turn on the logs and try to login again?

• How do I turn on sync logs:

1. Open about:config and turn on: services.sync.log.appender.file.logOnSuccess and services.sync.log.logger.engine.bookmarks
2. Restart Firefox and look at: about:sync-log
3. Logs and error messages will be located here

Please also use pastebin.mozilla.org for sharring any errors that relate to this issue.

Thank you.

Can you access the account via this page?

• https://accounts.firefox.com/settings

Try to delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave). If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

frenchredfrog said

Nothing to add. Three days I wait for connecting on https://accounts.firefox.com/signin?service=sync&context=fx_desktop_v1&entrypoint=menubar

Yes, it's ok. Thx!