How to Set Up and Use a Firefox Proxy for Better Privacy? or Is a Firefox Proxy the Best Way to Brows

This might sound dumb, but is your system clock correct? SSL/TLS proxies freak out if your time is off by even a few minutes.

Also, try launching Firefox in "Troubleshoot Mode" (help menu) to rule out extensions breaking things.

If you’re using a paid proxy, their support might have a config guide—worth asking!