The full details are in
https://seclists.org/oss-sec/2023/q4/132
but the gist is that Firefox allows arbitrary pages to change to content of your primary selection buffer. 

Patch to prevent this would be:
diff -r 9b362770f30b layout/generic/nsFrameSelection.cpp
--- a/layout/generic/nsFrameSelection.cpp       Fri Oct 06 12:03:17 2023 +0000
+++ b/layout/generic/nsFrameSelection.cpp       Sun Oct 08 11:04:41 2023 +0300
@@ -3345,6 +3345,10 @@
     return;  // Don't care if we are still dragging.
   }

+  if (aReason & nsISelectionListener::JS_REASON) {
+    return;
+  }
+
   if (!aDocument || aSelection.IsCollapsed()) {
 #ifdef DEBUG_CLIPBOARD
     fprintf(stderr, "CLIPBOARD: no selection/collapsed selection\n");

There's a lengthy discussion on the mailing list. In my opinion that's very undesirable behavior and we should carry the patch if upstream doesn't want to change