Fix thread safety issues with tabs collection

[data-sync-user]

Our tabs array is not thread-safe, however there are a few places where we are accessing this collection off the main thread where potential concurrent read-writes are possible.

Action item: investigate a solution, whether that is a synchronization primitive to make tabs thread-safe or updating the callsites using Task to access tabs in a background thread to be MT-only.

Examples:TabManagerMiddleware.closeTabFromTabPanel() which calls into closeTab, and TabManagerImplementation.restoreOnly()

Possible related crashes: https://mozilla.sentry.io/issues/5504377291/events/5ffe33f5ac604e4ba519b02ff058c538/?project=6176941&referrer=previous-event

┆Issue is synchronized with this Jira Task