By default, Firefox has a
Share Selection contextual menu (right-click) item. Between the
Search Provider for "..." and the
View Selection Source options when highlighting text.
Searching about this feature reveals a mixture of bug reports and, thankfully, advice to remove this by moving the Share This Page item from the menu into the additional tools are. However, there is no information about what it actually does.
I only searched after accidentally clicking it, so what did it do? Did it send the highlighted data elsewhere?