As of version 102 I intend to remove window.sidebar. This has only been supported by Gecko.
Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=1768486
Pref: dom.window.sidebar.enabled
https://bugzilla.mozilla.org/show_bug.cgi?id=1717612 already disabled it on nightly/early beta 11 months ago, and since then we saw no regression but a webcompat case where disabling window.sidebar fixes the issue. (https://bugzilla.mozilla.org/show_bug.cgi?id=1768345)