Reproduced the issue with Firefox 106.0a1 (20220916213956) on Windows 10x64. Opening the select menu in the attached test case from comment 0 will display the scrollbar on the right side. The issue is verified fixed with Firefox 107.0a1 (20220921214338) 106.0b3 (20220922092933) from comment 10 on Windows 10x64, macOS 11 and Ubuntu 20.04. The select menu from the attached test case has the scrollbar on the left.
Bug 1791219 Comment 12 Edit History
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Reproduced the issue with Firefox 106.0a1 (20220916213956) on Windows 10x64. Opening the select menu in the attached test case from comment 0 will display the scrollbar on the right side. The issue is verified fixed with Firefox 107.0a1 (20220921214338) and 106.0b3 (20220922092933) from comment 10 on Windows 10x64, macOS 11 and Ubuntu 20.04. The select menu from the attached test case has the scrollbar on the left.