Why should "Ctrl+alt+s" work? (Maybe that is browser or OS specific?)
Hovering the mouse over the "Submit changes" button states "Alt+Shift+S" in my Firefox 41 on Linux.

Sorry, made a "typo".

• For Chrome Alt+S works to save
• For Chrome on Chromebooks Alt+Shift+S brings up settings
• For Firefox (on Windows) Alt+S brings up History-tab in the Alt menu
• For Firefox (on Windows; up to a few versions ago, 1-2 stable versions?) Alt+Shift+S used to work to save, now does the same as Alt+S above. Something changed.

Alt+Shift+S still works to save here, but running older Firefox 41.

So I'd also guess this is an upstream issue. Cannot see any obvious reports in upstream within the last 60 days: https://bugzilla.mozilla.org/buglist.cgi?chfieldfrom=-60d&longdesc_type=allwordssubstr&chfieldto=Now&component=Keyboard Navigation&product=Firefox
Do you plan to report that to upstream?

In T114793#1708490, @Aklapper wrote:

So I'd also guess this is an upstream issue. Cannot see any obvious reports in upstream within the last 60 days: https://bugzilla.mozilla.org/buglist.cgi?chfieldfrom=-60d&longdesc_type=allwordssubstr&chfieldto=Now&component=Keyboard Navigation&product=Firefox
Do you plan to report that to upstream?

Already done, see description for link.

Patch: https://hg.mozilla.org/mozilla-central/rev/1b02f9d6c058