It could be a problem with Flash or another plugin. You can try to empty the Flash cache via Windows Control Panel > Flash Player > Storage > Delete All. Likewise for other possible plugins that have a control applet either in the Windows Control Panel or Start menu. You can also try to ascertain the site requirements from them. On the other hand if it is a site issue, it would be best to contact the site about their requirements, and also perhaps request them to make it compatible across all browsers.

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

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