All the previous suggestions could work for you, depending why you need to switch. Tor is a somewhat more secure browser but has been known to attract the attention of the "Powers that be".
I'm a long term Firefox user, from the Phoenix beta before it was renamed Firebird and then Firefox (isn't copyright wonderful? Not!). But I'm on the verge of giving up on it. I hated the Australis GUI they forced upon us and worked through the settings to restore the look and feel of the GUI I preferred. This is much easier now with the ClassicThemeResorer add-on. But at version 42, which has just become the stable release, it is no longer possible to get back the pop-up options window - the InContent flag is no longer honoured and the substitute New Tab is terrible. And I didn't bother with more than a quick look at Developer Edition 44 - yeuk!
But what I am looking at is Pale Moon, which is a fork of Firefox immediately before the Australis mess. It has the classic Firefox look and feel and the developer applies all the Firefox patches (except Australis, obviously) from the base code. It is stable, slick (like Firefox used to be) and runs on Windows, Linux and Mac. It's open source, so free, though the developer is open to voluntary donations if you wish. From what I've seen so far, I think I might go that way.