In Firefox 53.0, having a multi-row tab bar enabled on the top or bottom, with enough tabs to need more than one row, causes three issues that I've seen.
Sluggish page and mouse response, with every additional tab causing more slowdown
Flickering between a single-row and multi-row tab bar
The mouse cursor hot-spot is offset by a distance equal to the size of the additional tab bar/s below the actual cursor.
Here's a cropped screenshot of point 3:
In the attached image, the red square is the distance between the top of the letter "i" and the top of the mouse cursor in the single-row set, and is duplicated for consistency. In each set, the mouse hot-spot is at the very top of the link. The blue rectangles go from the top of the red square to the top of the cursor, and match perfectly with the additional rows of tabs.
In Firefox 53.0, having a multi-row tab bar enabled on the top or bottom, with enough tabs to need more than one row, causes three issues that I've seen.
Here's a cropped screenshot of point 3:
In the attached image, the red square is the distance between the top of the letter "i" and the top of the mouse cursor in the single-row set, and is duplicated for consistency. In each set, the mouse hot-spot is at the very top of the link. The blue rectangles go from the top of the red square to the top of the cursor, and match perfectly with the additional rows of tabs.