Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

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

You can check for issues with the places.sqlite and favicons.sqlite databases in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry.

@cor-el Thanks but your advice is irrelevant. And there is no "Verify Integrity" button.

@FredMcD What you wrote does not make sense. I only have an antivirus software. And I don't need to add any exception for Firefox. And AV software I use does not matter. I used to use Avast, now I am using ESET and the problem with Firefox has been the same regardless of an AV solution...

And did you read that the problem lasts < 15 seconds? If it was pernament I would have resolved it myself.

And I have performed another test. And this problem occurred only in Firefox, not in Chrome. So it proves what I had written before.

If your places.sqlite database file (bookmarks and history) is quite large then it can take some time to process this file as this is done lazily in the background. During this time Firefox may behave differently and not respond to actions that require checking this database.

"Verify Integrity" can be found further down on the "Help -> More Troubleshooting Information" (about:support) page under "Places Database".

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

Sorry for my late response. I have verified the integrity and the database is coherent. It took less than 4 seconds so I do not think that here lies the issue I described.

> Task: vacuum + Initial database size is 46080KiB + The database has been vacuumed + Final database size is 46080KiB > Task: stats + Places.sqlite size is 46080KiB + Favicons.sqlite size is 16416KiB

This is not about places.sqlite being corrupted, but merely about its size and the number of history visits and bookmarks stored in the file. The size being around 46 MB means that the file is quite large and Firefox might take more time to process history and bookmarks.

It is also possible that your security software is causing problems.

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

Sorry for late reply. But restarting and restoring system is a long process that I perform only when really necessary. I booted the Safe mode. But the safe mode does not have power management. So of course it was impossible to test my issue. Regarding security software, I used software from two different vendors and problem was the same.

. . . . restarting and restoring system is a long process . . . . .

You were not asked to restore your system. Only to try the computer's Safe Mode.

As to your security software, sometimes they get in the way. Safe Mode could check for that.

You did not even (wholly) read what you were replying to. So farewell. I see this will stay unresolved forever. I tried my best but I wasted my time. As I hoped that a developer would respond. Feel free to close this question.