Yes, SeaMonkey is the successor to Netscape and is called the Suite.
You don't have to reinstall to use a different profile but you will lose your current profile. You could just keep using it where it is but if move to AOS, you will likely lose your profile, whereas if you have it standalone, you can move it to a new install with the same MOZILLA_HOME setting.
The trick to getting OS2World to load is to change the user agent. I attach the user agent switcher extension. Unzip and drag'n'drop the XPI on the browser window, say yes to installing, restart if asked.
Afterwards, under Tools there will be a menu setting, Default user Agent, click it and go to Edit User Agents.. Click New, click User agent and use this, "OS2zilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 SeaMonkey/2.53.23" as the user agent (without the quotes) put OS2zilla as description, perhaps fill out the rest.
Now you can got Tools-->Default Agent and change it to OS2zilla and some sites will now work, lots won't too. Seems you have to do this every time you start the browser.
There's lots of extensions out there, somewhat hard to find the ones that work with our old browser. Things like UBlock_Origin make a better browsing experience.
Tools-->Addons Manager