Testing With Firefox
When at the "untrusted" page, and "continue anyway" doesn't work. Use CTRL+F5, then "continue anyway" and that should work. If you can't see the "continue anyway" use the below expert_bad_cert setting.
If you're testing with firefox and you're using self-signed SSL certs you'll see an "untrusted connection" dialog and have the option to "continue anyway". If you're going through a proxy, you sometimes won't get the option to "continue anyway". Here's some tricks that might help.
In tab1 go to the url that causes the error. In tab2 go to about:certerror. Copy the url from tab1. In tab2 click "continue anyway". In the dialg for adding an exception paste the url from tab1 and click "get cert" then store the exception. Restart the browser.
Go to about:config. Ignore the warning. Ensure that browser.xul.error_pages.enabled is true (the default). Change browser.xul.error_pages.expert_bad_cert to true. This will cause the "continue anyway" option to always be open. Restart the browser.
Clear your cache, history, cookies, etc. Restart the browser.
I'm not sure which of the above solved my problem. Eventually I hit be bad page, then entered an unrelated page, then pressed the back button, and the bad page displayed correctly. After a few CTRL+F5 it worked consistently.