Repeated login prompts drove me crazy during one proxy test because the credentials were correct, yet Firefox kept asking for them again. Restarting the browser helped only briefly. I eventually suspected that either the saved authentication data or the proxy itself was causing the loop. What are the usual reasons Firefox keeps showing proxy login windows even after the username and password have already been entered?
Repeated prompts usually make me check authentication before anything else. Old saved credentials, an incorrect username or password, or a server that is rejecting the account can all look similar from the browser side. Testing another server with the same Firefox profile helps narrow it down. If that works normally, I know the browser itself is probably fine and the issue is tied to the original proxy or its credentials.
I had this happen after changing proxy credentials while Firefox still had an older login saved. The first connection worked inconsistently, then authentication prompts kept returning on new tabs. I went back through the firefox proxy (https://www.simplynode.io/post/how-to-set-up-and-use-a-proxy-in-firefox) configuration, confirmed the host and port, cleared the outdated saved credentials, and restarted the browser. After entering the current username and password once, the prompts stopped. On another occasion, the problem came from a server that was rejecting the account intermittently, so testing a second endpoint helped separate browser trouble from proxy trouble. Since then, I always verify credentials and server availability before changing unrelated Firefox settings. That usually saves a lot of unnecessary troubleshooting.