The mouse seems to work fine. When this happens, I still have the ability to scroll in MS word documents, Excel spreadsheets PDF files and the like.
Firefox has been pushing out updates so fast that many software applications become incompatable until FF tweaks the application.
Your mouse-ware may be experiencing some incompatability issues. Check the latest list from FF (usually given when you update) or contact Mozilla for help.
Those tiny little wires do break. Do you have an old mouse to swap?