I'm a long-time Firefox user, but have recently been using a clone (may be incorrect term), PaleMoon, when I work at WikiTree.
I just tested the RootsSearch feature on Firefox Quantum. It worked fine after I made an adjustment (to allow a WikiTree script; https://apps.wikitree.com/apps/york1423/rootssearch/) in my script blocker addon (NoScript). Before I made the adjustment, RootsSearch had this message:
Unable to load the person. Check the ID and try again
If you use a script blocker, your problem may lie there. But I'm not a Tekkie, so it could be another problem!
I did notice that my script blocker was not present when I first updated to Quantum. Only after the script blocker itself was updated did it appear and function.