I understand that this may be an external problem not related to the browser, for example a virus, and the browser is set in the settings as the main one by default. Let's not take that into account. Could this problem be related to the browser? In this case, how to find and fix it?

What does the message in your screenshot say?

for example a virus </blockquote>

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.