Add locale: sv_SE

Add locale: ru

Update locale: zh-TW

Add position options: left adjacent, right adjacent
Now the relations "parent tab" and "last accessed tab" can be combined with position options such as "left" and "right". To keep compatibility, if these relations are used in previous versions, the position options will be set to "first".

Add option: Operating mode
I've been investigating some recent bug reports. If you are having problems on the latest Firefox version, try setting Operating mode to v4 and see if it improves.

Add relation option: pinned tab, unpinned tab

Improve option: Fix incompatibility with switching to tab from the address bar (requires Firefox 65 or later)"

Add option: skip unloaded

New APIs will be used to mitigate flashing, Ctrl+Tab order changes and tab discarding incompatibility. Hopefully, it will be completed in Firefox 65 (to be released on 2019-01-29).

Add relation: unread sibling tab
Add option: skip hidden tabs
Add option: exclusions

New APIs will be used to mitigate flashing, Ctrl+Tab order changes and tab discarding incompatibility. Hopefully, it will be completed in Firefox 65 (to be released on 2019-01-29).

Add relation: unread sibling tab
Add option: skip hidden tabs

Fix a problem that wrong tabs are selected if "Undo Close Tab" is previously used.

Add shortcut support.

Add option: sibling tab, child tab, unread tab, unread child tab.
Allow combinations of tab positions and relations.
Improve performance for Firefox 57 or later. Firefox 56 or earlier versions are affected by Firefox's Bug 1366290, and a mitigation option is added.
Add option: disable if the closed tab is a newly created about:newtab.