It's not a huge issue or anything serious. I have been a Firefox user for as long as i can remember and for years i have noticed these cursors appearing at weirdest places. you click somewhere on screen at new tab or in setting a cursor would appear. This issue has been there for ages. I don't know why but i get irritated seeing cursor appearing on Firefox logo on new tab. I can only laugh when i see a cursor somewhere i could've never expected.