You can check the timing in the Network Monitor.

• https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Problem still happens in troubleshoot mode. Have also tried this on another PC with a fresh install, and still has the problem.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

Are those websites possibly using local storage (indexedDB) as that might be disabled in PB mode ?

Here are the sites. These sites are all mine, so I can answer any questions about them.

Load these in Firefox Private Mode to see the loading delay.

This site loads quickly, no problem: https://gator1.tech/

This is the same web site content as above, but has the 4 second delay problem: http://damondb.org/

Here are 2 more test sites that also have the delay problem: http://fortunadb.org/ and http://fortunadb.net/

both of these have about 4 second delay when accessing with Firefox in Private Mode.

The above web sites are just HTML/CSS no databases, content management systems, etc.

fortunadb.org and fortunadb.net are just simple single-HTML page sites.

The other thing I notice is that I have never seen the delay problem with https sites, only http sites (no secure server).