Very, very annoying. Frequently occcurs when attempting to make an online transaction - which can take numerous attempts to complete due to this issue. Have used Firefox for very many years, switched back to Mac last year (June), all OK until very recently when this problem arose, shortly after an update I believe. Only occurs when using Firefox, Safari is stable. Want to continue with Firefox as default browser but will change to Safari if this can't be resolved. Has this been experienced by other users - Mac users?