I have a setting which requires me to approve of all links before going there, so I am not permitting RNDDMN to "get a hold of me". I've noticed that it presents as an additional tab on my FF, usually when I click a different link.

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I ran all the scanners listed but I'm still getting hijacked occasionally. Funny I find no references to RNDDMN.com by searching the web.

That means it was a stealth install and your A/V no longer can do anything about it since it got through protection. Consider taking to computer shop to remove the stealth malware program.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

If it still happens, then this is caused by another program on your system.

Check the programs that are on your computer Windows: Start > Control Panel > Uninstall Programs.

Sort the list so that the latest program installed is at the top of the list. Remove anything that you don't know what it is.

Keep us posted.