Google has invested massively into security. On various platforms (non-Chromium Linux excluded), Google Chrome uses advanced defence-in-depth that make Chrome much more secure than Firefox on the same machine. Their origin-based process separation make Chrome a memory hog but protect tab processes from each other in a way Firefox doesn't bother with just yet.
Chrome may be a privacy nightmare, but in terms of security it beats Mozilla.
Defense is not very meaningful if your browser is provided by one of the parties you need to defend yourself yourself _against_.
Same could be said about Windows vs Linux back in the day, but as another person already pointed out it doesn't make sense when the owner is one of the ones you are trying to protect yourself against.
Also, as it turned out, Windows wasn't much more secure than Linux, and I guess we'll find this with Chrome as well. In fact I wonder if this isn't obvious already now that uBlock Origin doesn't work on Chrome any longer?
Besides, isn't Chrome approaching 20 years now and I still cannot have tree style tabs on it so it is still a toy browser meant for causual browsing, not work ;-)