Firefox won when it was significantly better than IE in ways that lots of people actually cared about (it had tabs). Then Chrome won when it was significantly than Firefox in ways lots of people cared about (it was fast and if a tab crashed the whole browser didn’t crash with it).
For a long time, no browser has been significantly better than Chrome in ways lots of people care about. I don’t know if Manifest v3 is as bad as everyone says, but if it is, that might open up a big enough point of difference.