It's been months now. I thought it was probably a known issue or a conflict with any of my extensions but I still haven't figured out a way to solve it. Have tried re-installing the browser, I even switched to a new linux distro and everything was reset, I still have the issue. The problem is - everything works but the ability to select/copy multiple blocks when using WordPress editor (tested it on three sites with the latest WordPress version where I write) does not work only when utilizing the mouse/cursor. However, it works when using Brave or Google Chrome. In either case, when I try to select multiple text blocks using '''CTRL+Shift+any arrow key''' on Firefox, it works no matter what. But, using the cursor/mouse, I can't achieve doing that...