For my daily driver & primary work computers, Firefox is what I prefer to use for all of my browsing when possible. But, under certain circumstances (most commonly wanting to login to a second web account simultaneously without logging out of the first), I'll use Chrome as my "Plan B" browser. Also, I've noticed that Chrome seems to better handle long-duration livestream videos than Firefox, so if I ever notice Firefox starting to get bogged down by the livestream or the video repeatedly stops to buffer, I'll switch it over to Chrome.
On the side, I perform freelance computer repairs, usually via house call. If I have determined that the customer's default browser is not compromised (infected with malware and whatnot), I'll use that because doing so allows me to make sure it is functioning properly. If it is NOT working properly, or the computer is infected (in which case I'll be using the computer from a known-clean Windows PE bootable USB), Firefox Portable is my Plan B. I also have a "just in case all other options fail" Plan C browser on my utilities USB, a very small and admittedly quite old browser called OffByOne. If for some reason I can't run any other browser, I can use that to do basic Google searches and file downloads to try and get the system back to a somewhat-normal level of functionality so I can switch back to Firefox Portable to continue my troubleshooting.
When it comes to my smartphone running Android Pie, I primarily use Chrome because it feels (to me anyway) to be the fastest and also works best with a specific website I use at work for online ordering. But, I also have Firefox installed as an ad-free alternative since I have the ABP extension on it.