How can I solve the Firefox HSTS error?

For solving the Firefox HSTS error, you can follow the below steps -

• Type about:support in firefox
  
• Click Profile Folder | Open Folder which should open your profile folder.
  
• Find file called SiteSecurityServiceState.txt and open it
  
• Find the entry for your site url and remove it. Entry would looks something like - github.com:HSTS 120 17242 1521194647604,1,1
  
• Make sure the above firefox is closed so that it does not overwrite it.
  
• Firefox stores HSTS entries in this file with their expiration periods. Removing this entry should allow you to hit http url. TO further prevent it you can probably change the permission of this file to read only.
  
• More details - Understanding HTTP Strict Transport Security (HSTS)
  

NOTE : This will not work for well known sites like google as those lists are preloaded by browsers. Works fine for others. See above link for details.