Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

I always see this message if the page is not displayed. Please see the attached image. First time that I realized that, was when I tried to visit the page nikas.tech

From the logs of the server I can see:

[client 37.6.31.52] ModSecurity: Access denied with code 403 (phase 2). Matched phrase "Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0" at REQUEST_HEADERS:user-agent. [file "/etc/httpd/conf/plesk.conf.d/modsecurity.conf"] [line "83"] [id "980001"] [rev "1"] [msg "Tophost Bot Rule: Black Bot detected."] [severity "CRITICAL"] [hostname "--myhost--.gr"] [uri "/"] [unique_id "YUiz-RjDSf8V6dzZdYt1KwAAAIM"]

I don't know about this and I called for more help.

Make sure you are not blocking content.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

Thank you for your help.

I'm not blocking content. I have no firewalls or other relative applications. It's a fresh Debian Stable installation. I've already tested FF safe mode with no result. I have already try to Turn Off the Enhanced Tracking Protection for these websites. Problem remains in Private Browsing as well. I tried not only to clear FF cache using FF's menu, but also switched off FF, and renamed /home/myusername/.mozilla and /home/.cache/mozilla folders, where in Linux profile and cache is stored.

As for SmartBlock, I followed your link, but in my configuration editor there isn't "extensions.webcompat.enable_shims" preference.

Finally, I upgraded FF to 93.0 from Debian unstable repositories, and it's all fine. So??? Seems it was a version problem, right? Should I fill a bug report?

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.

As to a bug report, if the new upgrade is working, all is well.

Solved but not suggested:

Solved in the next Firefox release (93.0).

Please be aware that Firefox 93 for .deb based distributions is depended on libc6 2.32-4, while Firefox 87.xx is based on libc6 2.31-14. Using libc6 libraries (fundamental for all Linux distributions) from Debian unstable in a Debian stable installation, may cause serious dependency breaks in future.

For me, I hope that within the next 2 months, all packages marked now unstable, will be delivered in stable repositories. In case somebody looks for a solution, If I face any trouble, I'll let you know.