I use Firefox everywhere.
Originally because I’ve been using the Internet for a long time. I started with this product back when it was Netscape Navigator and my choice was either that or go back to NCSA Mosaic.
Later, when Netscape became Communicator and incorporated e-mail, I used it for that as well.
Later on, when Microsoft was trying to push everybody into Internet Explorer, I dug in my feet and refused to switch, because of the massive security problems in their software at the time.
Today, I continue using Firefox because it is the same app on all the operating systems I use (Windows, macOS and Linux), it has pretty good security and privacy features, and they have a sync service that lets me share my passwords, history and bookmarks across all of my Firefox installations on all platforms.
I don’t have any particular objections to others (aside from Chrome - I don’t trust Google), and I have installations of Edge (in order to have something Chromium-based) in addition to Safari on my Mac. But I rarely use them - usually only if there’s some web site that doesn’t work well with Firefox.