This addon lets you correct one of Firefox's bigger usability problems: the unpredictability of which tab will be selected when you close a tab. It also does so with the minimum necessary permissions, which in an age of spying browser addons is something to appreciate.
Firefox keeps some internal info about which tabs are 'children' of 'parent' tabs, and depending whether you have clicked around in the tab bar or not, may choose to activate some 'peer' of the closing tab rather than one adjacent to the closing tab. But this information is not communicated to the user at all, the tab bar appears to be just a list of tabs! I'm fairly technically literate and I find this behaviour stunningly confusing, I can only imagine how frustrating this must be to less technical users. I'm not even sure that this description of how it works is 100% right, and I've spent some time looking into it!
In my opinion, Firefox's closing order for tabs without parent-child relationships (selecting the tab to the right) is also wrong. When you open a new tab, it is opened on the right. It only makes sense that closing a tab would select the tab to the left, taking you back toward where you began. The fault doesn't lie entirely with Firefox though, this is also Chrome's behaviour, and the Firefox team must feel much pressure to keep their interface consistent with the competition to make switching easy.
But I digress. These are small user interface issues, but minor details like this can have major impact on the user experience. Kudos to the author for making this extension.
The only flaw is that, when configured to select the tab to the left, there is a brief flash where the tab to the right is selected before switching to the tab to the left. I presume that this is due to a limitation of the current extension API, where the extension reacts to the tab closed event after the tab is gone and Firefox has already made its default next tab selection, and that nothing can be done to fix it. Unfortunate, but not a deal breaker!
Users who care about tab opening behaviour are also encouraged to check out an about:config option added in Firefox 60, which positions new tabs to the right of the opening tab (instead of to the right of the current peer tabs, or at the end of the tab strip entirely):
browser.tabs.insertAfterCurrent = true