What's the day! I just found another life-changer life hack. When you copy something styled and then paste this to some WYSIWYG editors it copies the styles as well. This styling usually is not needed, so you need to paste this text to Notepad first and then copy it once again before pasting to clear the styles or use the context menu and select "Paste as plain text", that is also not so comfortable as just pressing Ctrl+V. Today, I googled how to turn on "Paste as plain text" by default in Firefox and found that most of the OS supports hotkey for this, and it's "Shift+Ctrl+V". Omg, I suffered for years, but it turned out to be so simple.