A Windows user [https://support.mozilla.org/questions/1119085 reported a similar issue with a related site] yesterday. Infections that affect both Windows and Mac usually are add-ons. Could you try this: (1) Open Firefox's '''Add-ons page''' using either: * Command+Shift+a * "3-bar" menu button (or Tools menu) > Add-ons In the left column, click '''Plugins'''. Set nonessential and unrecognized plugins to "Never Activate". In the left column, click '''Extensions'''. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. ''All extensions are optional; none come with Firefox.'' Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step. Any improvement? (2) To check for external software that might reinfect Firefox, run a sweep of your system with Malwarebytes Anti-Malware for Mac (free version): https://www.malwarebytes.org/antimalware/mac/ Can you get rid of it?