Vertically scrollable OOUI toolbar popups have unnecessary horizontal scrollbar on Firefox.
I've fixed this issue already once in 2014 (T67059), it looks like it regressed due to some recent change in Firefox.
For example, on https://doc.wikimedia.org/oojs-ui/master/demos/?page=toolbars&theme=wikimediaui&direction=ltr&platform=desktop, click "Insert" then "More" in the second demo:
| Firefox | Chromium (MS Edge) |
|---|---|
This also affects the visual editor toolbar:
| Firefox | Chromium (MS Edge) |
|---|---|










