The zip file I downloaded was 143.0.2. But after running it, the about box said 143.0.3.
I just wrote about this in another thread: Discourse drops support for old versions of Firefox - #8 by Shamino
You can run both on the same computer. In my case, I renamed the app to “Firefox Dynasty” in order to avoid confusion. Launchpad only shows one icon - I think it got confused, because I didn’t rename it in any of the resources inside the App bundle. Which I won’t do, because I don’t know if it might break anything.
When I ran it, it created a new profile and went through the usual first-launch set of screens. I chose to not import anything.
If you want to just run with that and configure it (install add-ons, etc.), that should be just fine.
In my case, I made a backup of my original ESR profile before starting, just in case something failed. But that turned out to not be necessary.
I replaced the entire content of the new profile directory (used by Dynasty) with a copy of the content from the old profile (used by ESR), in order to see if everything would just migrate. That seems to have worked for me.
After a couple of days, I decided to delete my installation of ESR, but I am leaving its profile in place, in case I want to go back for some reason.