Submitting an inline comment I intended to submit yesterday...

Thanks Itiel, I should have realized RTL needs updating too.

I think you're right that it's intended to be nested with the rest of the background rules, and AFAICT are the only actions with background images. Whether or not it's nested turns out not to matter though because it still applies to STT (and so the 4px *should* be updated to ). However, since nesting does seem to be the intention, I'll re-nest it.

Details:

I did some archeology on the RTL line. It was added in bug 1918437 D223704 for scotch bonnet. It was originally nested like this:

/* Switch-to-tab and Clipboard action text is styled as a chiclet. */
.urlbarView-row[has-action]:is([type=switchtab], [type=remotetab], [type=clipboard]) {
 > .urlbarView-row-inner > .urlbarView-no-wrap > .urlbarView-action {
 @media (-moz-bool-pref: "browser.urlbar.scotchBonnet.enableOverride") {
 /* background-image and other background rules set here */
 &:-moz-locale-dir(rtl) {
 background-position-x: right 4px;
 }
 }
 }
}

See: https://searchfox.org/firefox-main/rev/522658f297db18d5a00a1f9f01be96e4017fc7ee/browser/themes/shared/urlbarView.css#716-753

The rule was added later in bug 1908451 D231168, which un-nested the RTL rule from the rest of the background rules. That revision did not introduce background images in other non-switch-to-tab cases, so it looks like the un-nesting was unintentional. By that time the RTL rule had become slightly separated from the rest of the background rules by the addition of , so maybe that's why.

Re-nest the RTL rule

Getting these several revisions lined up for landing, rebasing on Jules's patch