Scroll bars that are impossible to remove seem to be appearing on page navigators, slicers, cards, etc. on Power BI Service. This happened sometime July 24-26th.
Yeah it's a firefox update, Firefox 153.0, See All New Features, Updates and Fixes
Bug filed on them. But we are trying to handle it in our code gracefully also. No ETA unfortunately, may take a few weeks to get this out.
2057397 - Regression: Firefox 153 changes scrollbar behavior for websites using webkit scrollbar CSS