Firefox needed so long for a multi-process architecture because Mozilla focused on other things, like developing mobile operating systems. The main browser was

Firefox needed so long for a multi-process architecture because Mozilla focused on other things, like developing mobile operating systems. The main browser was kept in hibernation mode. Also, as the poster said, Firefox had a rich Add-on API that they wanted to keep support for but which they eventually decided to drop. The new API isn't close to the versatility of the old API, sadly some add ons work worse now. A better example would be I think iOS parts of which can't adopt swift and have to stay implemented in ObjectiveC because of binary compatibility concerns.