Reproduced the issue with Firefox 95.0a1 (2021-10-26) on macOS 10.15 using attached tc's from comment 0 and STR. The custom cursor is exiting the Firefox window if the cursor size is changed from macOS preferences. The issue is fixed with Firefox 95.0b5 (20211109194756) on macOS 10.15. After changing the cursor size and restarting Firefox, the custom cursor is no longer exiting the Firefox window.
Bug 1737751 Comment 8 Edit History
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Reproduced the issue with Firefox 95.0a1 (2021-10-26) on macOS 10.15 using attached tc's from comment 0 and STR. The custom cursor is exiting the Firefox window if the cursor size is changed from macOS preferences. The issue is fixed with Firefox 95.0b5 (20211109194756) on macOS 10.15. After changing the cursor size and restarting Firefox, the custom cursor is no longer exiting the Firefox window. Per comment 4 the need to restart Firefox for this fix to work is a known issue.