Firefox' scroll behavior seems to be non-deterministic on Linux, so there doesn't appear to be a universal answer. The default is 100, which seems to work fine

Firefox' scroll behavior seems to be non-deterministic on Linux, so there doesn't appear to be a universal answer. The default is 100, which seems to work fine most of the time, but for the OP's issue, reducing it by a factor of ten might be a good starting point.