In PB a separate cookie jars is used and the disk cache in disabled.

So the advice would be:

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

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

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window
• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Another feature of private windows is that Firefox automatically blocks content from certain sites associated with tracking. This tracking protection feature -- if it is kicking in on the site -- may be blocking something that is causing the problem in the regular window. Do you see the "shield" icon toward the left end of the address bar when you visit the site in the private window?

More info on this feature: What happened to Tracking Protection?

jscher2000 said

Another feature of private windows is that Firefox automatically blocks content from certain sites associated with tracking. This tracking protection feature -- if it is kicking in on the site -- may be blocking something that is causing the problem in the regular window. Do you see the "shield" icon toward the left end of the address bar when you visit the site in the private window? More info on this feature: What happened to Tracking Protection?

Yes, I do see the "shield" while visiting the site in the private window. However, how could that cause a problem in the regular window?

I've done the cookie and cache clearing mentioned in cor-el's answer above, which didn't help, so will try the Safe Mode next.

If clearing cookies didn't help then It is possible that the cookies.sqlite file in the Firefox profile folder that stores the cookies got corrupted.

• rename/remove cookies.sqlite (cookies.sqlite.old) and if present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder in case cookies.sqlite got corrupted.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can check the Web Console for error messages about blocked content.

• https://developer.mozilla.org/Tools/Web_Console

mike_the_wino said

Yes, I do see the "shield" while visiting the site in the private window. However, how could that cause a problem in the regular window?

I was thinking of it from the other way around: maybe the tracking protection is helping. As a test, you could click the shield icon and turn off tracking protection on the site in the private window and see whether the site breaks.

So, I tried Safe Mode, and the site pages load normally...no blank screen. Does this indicate one of my extensions is causing the problem? The only ones I have active are Adblock Plus and LastPass, and they were active before the problem started.

I haven't looked into the potentially corrupted cookies.sqlite file yet, but would this still be necessary knowing that I didn't have the issue in Safe Mode?

jscher2000 said

mike_the_wino said

Yes, I do see the "shield" while visiting the site in the private window. However, how could that cause a problem in the regular window?

I was thinking of it from the other way around: maybe the tracking protection is helping. As a test, you could click the shield icon and turn off tracking protection on the site in the private window and see whether the site breaks.

Yep, doing the test you suggested broke the site in the Private Window. What's up with that?

When Tracking Protection is actively blocking content then the Web Console will show a message about what content is blocked.

• https://developer.mozilla.org/Tools/Web_Console