Hey, I had a similar issue, and it turned out to be a conflict with another extension. Try disabling all your extensions and see if it works.
If it does, re-enable them one by one to find the culprit. Also, make sure your Firefox is up to date. Sometimes older versions have bugs with proxy settings Mozilla.
If it does, re-enable them one by one to find the culprit. Also, make sure your Firefox is up to date. Sometimes older versions have bugs with proxy settings Mozilla.