Fixed by https://hg.mozilla.org/comm-central/rev/ea72f328b8be (bug 1713130). Working on that bug, there were several places where RTL was assumed. So I don't think there's a quick way to get RTL to work in esr91. And there are several other patches in bug 1713130 that would need to be uplifted to get the fixes implemented there. So maybe the best fix for esr91 would be to just force the assumed `direction: ltr;` on the week and day views?
Bug 1712942 Comment 3 Edit History
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Fixed by https://hg.mozilla.org/comm-central/rev/ea72f328b8be (bug 1713130). Working on that bug, there were several places where LTR was assumed. So I don't think there's a quick way to get RTL to work in esr91. And there are several other patches in bug 1713130 that would need to be uplifted to get the fixes implemented there. So maybe the best fix for esr91 would be to just force the assumed `direction: ltr;` on the week and day views?