I personally think the reason was to further reduce the need to move your thumb from the bottom of the screen. The current iteration of safari is the only mobile browser I have ever used where I feel comfortable browsing and switching tabs with only one finger.
It's surprisingly well implemented, though some actions (such as accessing history or site settings) take a couple extra taps.
Probably doesn't have Firefox's infuriating tab manager that throws an idiotic "well golly you closed a tab!" popup over the next tab you intend to open.
They've fixed that on Nightly, for what it's worth. There is now padding at the bottom of the tab list. Also, undo-ing a closed tab puts it back where it was, not at the top of the list.
I've yet to try firefox mobile on my phone, I've had no reason to switch from safari (most of my browsing is on Brave on my laptop anyway), but I'll give it a shot.
Safari in iOS 15 also has extension support, and apps like Hyperweb[0] make the experience very pleasant.
The big challenge I have with this is that while the Safari bar is at the bottom, search within sites (say, Wikipedia) is still at the top. This ends up making it harder for me to redevelop muscle memory for searches/urls at the bottom
I think the purpose is (at least in part) precisely to "split up your muscle memory", so you don't use the same pattern for different kinds of searches. With browser search at the bottom of the UI, it's better differentiated from the Web site's search input at the top of the page.
It's surprisingly well implemented, though some actions (such as accessing history or site settings) take a couple extra taps.