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.
Status changed:
Investigating
to Accepted
Yes, Mozilla Firefox specific, based on this update: Firefox 153.0, See All New Features, Updates and Fixes
Workaround: use Edge or Chrome
We are seeing if we can do something on our end, but it may take a few weeks to roll out a fix.