STR:

1. Enable bidi on the toolbox (or otherwise use an RTL build)
2. Open about:preferences, scroll to the Website appearance section
3. Select the last (leftmost) option there, hit the Left arrow key

AR & ER:
Selection goes to the Light theme instead of Nightly's theme

This is a more general issue, afaict.

Test-case works ok in Blink, need to check WebKit. Indeed they reverse the forward / backward meaning for RTL: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/html/forms/radio_input_type.cc;l=135-137;drc=a46c11e44a13db20a366c86418679740516a4401

A spot-fix would be something like:

diff --git a/browser/themes/shared/preferences/preferences.css b/browser/themes/shared/preferences/preferences.css
index d8478c3f21ce7..95c49671e84e4 100644
--- a/browser/themes/shared/preferences/preferences.css
+++ b/browser/themes/shared/preferences/preferences.css
@@ -1482,6 +1482,11 @@ richlistitem .text-link:hover {
   gap: 16px;
 }
 
+#web-appearance-chooser:dir(rtl) {
+  /* Ensure that item order matches markup for key navigation to work */
+  flex-direction: row-reverse;
+}
+
 #web-appearance-chooser,
 #web-appearance-override-warning {
   margin: 12px 0;

Which would ensure markup order. But I think the Blink behavior is reasonable, and a bit less surprising.

Drive-by cleanups.

We have similar code for range inputs. This matches Blink (but not
WebKit), but I think it's a better default.

The spec seems to leave this up to UAs, so for now landing a
moz-specific WPT. Should be trivial to upstream if we want.

Depends on D141705

The patches are in DOM code so moving over there.

https://hg.mozilla.org/mozilla-central/rev/6c97b9e07bf5
https://hg.mozilla.org/mozilla-central/rev/41164ab04484

This works great now, thanks!