Beginning with Mozilla Firefox version 149, they introduced a new Open Link option. It is called Open Link in Split View. You may read about it here:

Split View in Firefox

I have found this recent addition annoying in that I end up accidentally clicking it instead of "in New Tab" or "in New Window" that I was actually intending. I went searching for a way to remove the new entry from the Right Click on a URL Link menu. Here is what I found:

Go into your Firefox configuration editor:

Search for entry:

Set the value of that entry to:

There you have it! No more Right-Click menu of a URL showing Open Link in Split View.