Strange, I found some information on that error in Firefox. This can happen if there are issues with the temp data from the site (cookies/cache) or if a proxy or VPN is in use. You might try to launch Firefox in Safe Mode to see if it still happens:
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
If you can load it in Safe Mode, then refreshing Firefox will probably fix it going forward. Just make sure to manually clear cookies after the refresh, as that is not part of the process.
https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings