Try the same page in another browser. If it works there, open Firefox Settings and check the proxy configuration. Compare it with the connection that works. On a personal network that does not use a proxy, try No proxy; on a managed network, check the required setting with IT.
01Find the proxy setting for your Firefox version
Section linkOpen Settings → Privacy & Security. Under Connection and software security, open Advanced settings, find Proxy settings, and select Configure proxy. Some versions put this under Settings → General → Network Settings instead. Before changing anything, note which mode is selected and keep a private copy of any address or port you may need to restore.
02Check where Firefox gets the proxy address
Section linkUse system proxy settings takes the configuration from your operating system, so check there if that mode is selected. For Manual proxy configuration, verify the hostname and port. If Firefox uses an automatic configuration URL, compare it with the one your network administrator supplied. If the dialog says an extension controls the connection, check with IT before disabling a required extension.
03Retest before keeping the change
Section linkOn your own network, if you know you do not need a proxy, try No proxy and reload the failing page and another site. If both work, the previous proxy setup was likely involved. Restore your recorded setting if the test does not help. If all browsers fail, check the shared network connection or proxy service rather than repeatedly changing Firefox.