FF58 Quantum on LinuxMint 18.3 is fine for me. When I open the site, this menubar is momentarily 'anywhere' on the page, then almost immediately jumps to its correct place. It always has done that for me, using FF/LinuxMint. I put it down to something in my overall installation – maybe you have a similar issue, but I don't have any other ideas, I'm afraid. Maybe try removing then re-installing FF Quantum?