I was using Firefox and had loads of bookmarks set up. I install Tor expecting it to be an alternative browser but it has taken over all my browsing. Can I get my old browser 'as-was' back? Thank you for any help.
The tor browser bundle is a standalone firefox, the bundle is run independent from any other browsers installed on the system.
If you downloaded the bundle, unpacked it and then run it from that folder then it will not interfere with other browsers on your computer.
I fail to see how it can 'take over', please elaborate on this if possible. (Where you downloaded from, how you installed it, how you run it.)
The Tor browser is a separate browser. You should be able to just start Firefox like you always did.
I often run firefox and TOR at the same time. They do not interfere with each other. Maybe you upgraded firefox without backing up your bookmarks first