@sgunhouse Hi, thank you for responding.
I always play with Opera open, listening to music on YouTube. I don't usually have many tabs open, at most 3, and I have 3 pinned tabs. But I noticed that even without any game open, sometimes just opening Opera, it already closes Steam.
I tried clearing the browsing data, and apparently, since yesterday, the problem has been resolved. I also scanned the PC with the antivirus to see if there was anything, but it didn't find anything.