"ajax" is a term used in website development to refer to a group of technologies. That message either means there's a problem with the website code, or maybe you're using a Firefox extension to manipulate the way the site behaves.

If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? To start in Safe Mode, go to Help > Restart with Add-ons Disabled.

If the problem does not occur in Safe Mode, then you can disable your extensions one-by-one until you find out which one is causing the problem. See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Thanks, but this also happens when I use Google Chrome.

Then it is likely a problem on the website's side :/

Make sure that your security software (firewall) isn't blocking such requests.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls