Yeah, agree that this behaviour doesn't quite look right on Firefox. I think this is related to bug 2002789 - We're positioning the fixed position element against `GetPosition()` value of the sticky element, which changes with scrolling.
Bug 2004627 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Yeah, agree that this behaviour doesn't quite look right on Firefox. I think this is related to bug 2002789 - We're positioning the fixed position element against `GetPosition()` value of the sticky element, which changes with scrolling, but then that position is in relation to the scrolled content, so that it looks stationary w.r.t. scrollport. Incorporating the sticky element's position in relation to scrolled content causes the fixed element to move by the scroll amount, whenever the fixed element reflows, in this case due to `:hover` changing `width`.