Over the last month or so Firefox has been failing on my old mac OS10.15.7. I go to my mac in the morning and firefox just won't start. I try to rebuild but holding down the option key when starting the app does nothing. I restart, sometimes resetting PR. That has helped in the past but doesn't now. I have re-downloaded and re-installed. I have deleted the firefox folder in library App support. As soon as one thing works it happens again after a day. I don't know what the hell is going on but I don't like it. I have run anti-virus software and malwarebytes. They are not picking anything up. I have another mac which uses up to date OS and that works fine. Is it a case of using a specific older version of firefox? I have tried an older version from another boot drive (startup disk) using OS10.13 once and it also only worked for a day or so.