Update: From Firefox 65, this extension can prevent the browser's default selection, so flashing, Ctrl+Tab order changes and tab discarding incompatibility are fixed. Firefox 65 is the current Developer Edition and will be released on 2019-01-29. Update: If there are problems with tab grouping extensions (e.g. Simple Tab Groups, Panorama View), see these compatibility options in v4.0. After closing a tab that is currently selected, Firefox usually moves the focus to the tab on the right. This extension can change this behavior and select another tab according to the preference instead. The options are:

Last accessed tab
Parent tab (Firefox 57 or later)
First/Left/Right/Last tab
First/Left/Right/Last unread tab
First/Left/Right/Last sibling/child/unread-child tab (Firefox 57 or later)

After installing you can configure this extension by clicking Preferences/Options of it in the Add-ons Manager. The buttons represent the order of preference: e.g. if "left tab" is at the top and "last accessed tab" is the second, after closing the current tab it will try to select the left tab, and then the last accessed tab (if there is no tab on the left). Click the up and down icons to modify the order. Known issues In Firefox 64 or earlier, the tab that Firefox selects by default may be selected and shown briefly.