3.8.17

* Attach tabs sent from other devices to the tree correctly, even if they are sent while the receiver Firefox is not running and `browser.tabs.insertAfterCurrent` is `true`.
* Put a set of tabs from other device together in a place instead of jumped positions if possible, when `browser.tabs.insertAfterCurrent` is `true`. (Set a secret option `fixupOrderOfTabsFromOtherDevice` to `false` when you want to deactivate the behavior.)
* Respect the right-to-left mark at the beginning of a page title.
* Better appearance of the counter for collapsed descendant tabs in the Sidebar theme.
* Don't collapse tree by double-click on tab closeboxes. It accidebtally collapsed the closing tree and closed all descendant tabs unexpectedly.
* Treat middle-click and accel-click actions on the "New Tab" command in the context menu on tabs same to actions on the "New Tab" button, in the sidebar.
* `<tab-item-substance>`, `<tab-label>` and `<span class="label-content">` now inherit `data-tab-id` and `data-window-id` attributes from the parent element (`<tab-item>`) for more customizability with custom user styles.
* Highlight tabs with "attention" class same as Firefox's native tabs even if they are not faviconized.
* Use highly-visible (darker) background certainly with the "High Contrast" theme on Linux.