If you're running two versions of Firefox - and TenFourFox is a version of Firefox - then you must use a different profile for each. You must also be careful to start the one that does not use the default profile with a '-P profilename' argument.

If, for example, your old Firefox is using the default profile and you start TFF without specifying its profile, TFF will probably use - and possibly update - the default profile. That may make it corrupt next time you use it for another version of Firefox. I've had this happen - profiles are not generally backwards-compatible.

To guard against mishap, backup the profiles. You can also, of course, backup just the bookmarks.

Mozillazine has technical articles on profiles, not always up to date.
e.g. http://kb.mozillazine.org/Creating_a_new_default_profile

Caveat - I don't know Macs and I've never used TenFourFox so any of the above may not be true in your case. But I often run multiple versions of Firefox and it's been the same for years. There;s probably a support forum somewhere if you need advice:
TenFourFox_development