Bug 1511613 "fixed" an issue where users desired tab switching on mousewheel scroll. This is a nice feature if users are expecting or are used to it, but has been removed from GTK3 (for example), and as such is not expected when using GNOME. GNOME Web no longer switches tabs when mousewheel scrolled over tabs, and ideally, Firefox would match. I don't have much experience with KDE, but this may be standard there, so I think the best course of action is to match platform convention based on the desktop environment that the user is running; just running on Linux, unfortunately does not provide enough information to Firefox what the actual user expectation is.
Bug 1512493 Comment 0 Edit History
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Bug 1285812 "fixed" an issue where users desired tab switching on mousewheel scroll. This is a nice feature if users are expecting or are used to it, but has been removed from GTK3 (for example), and as such is not expected when using GNOME. GNOME Web no longer switches tabs when mousewheel scrolled over tabs, and ideally, Firefox would match. I don't have much experience with KDE, but this may be standard there, so I think the best course of action is to match platform convention based on the desktop environment that the user is running; just running on Linux, unfortunately does not provide enough information to Firefox what the actual user expectation is.