Firefox Manifest V3 uses event pages, so the background script can restart with an empty in-memory snapshot cache. After restarting Firefox with multiple restored windows, a close event could arrive before the extension had a reliable pre-close snapshot for that window; in that case it intentionally did nothing and Firefox's default right-tab focus remained. The extension now initializes on browser startup, mirrors tab-order snapshots to session-scoped storage, restores that state after event-page restarts, keeps a larger history, and indexes the last snapshot seen for each tab so restore-event bursts cannot evict the closed tab's position before tabs.onRemoved runs.

In certain situations, closing a tab would activate the tab to the right rather than the left. This happened when the browser closed more than one tab at the same time — for example, when using "Close Tabs to the Right", closing multiple selected tabs, or "Close Other Tabs". The extension was assuming only one tab had been removed, so its position math could point at the wrong tab. It now finds the correct target by matching tab IDs directly against the pre-close tab order, which works reliably regardless of how many tabs were removed at once.