I wasn't able to do that even, Firefox with HSTS is dumb and goes against user freedom by failing to give a meaningful bypass method. Though I was able to get around it.

FYI people won't tell you how to disable HSTS enforcement, just delete the contents of the file SiteSecurityServiceState.txt then set permissions of it to read only and owned by root for good measure. Then the unbypassable warnings are able to be bypassed same as any other http warning on an https site.