Easy to use, no need to configure anything. No permissions required. Inspired by the old Tab Control add-on by Angly Cat that is not supported by Firefox anymore. Why does a wrong tab activate for a brief moment before the correct one does? This seems to be a limitation of WebExtensions technology (or at least its implementation by Mozilla). The browser first opens a tab at its default position and only then it lets extensions know a new tab has opened and gives them a chance to do something. Same thing when a tab is closed. If anyone knows of a workaround to this issue, please let me know on Github! Other known issues

The add-on conflicts with the built-in Switch to tab feature
Solved! Sometimes there could be a wrong tabs order when a lot are being open at the same time (eg by restoring a session or opening many tabs from bookmarks). Still having this one? Please let me know!

Are there any alternative add-ons? Yes, there are:

Tab Open/Close Control by F. Kolbe, that includes a lot of options, but sadly activating tab to the left on close was not working properly and the author hasn't released any updates for several months, which has prompted me to write this add-on.
Open Tabs Next to Current by Sebastian Blask, that does exactly what it says in its name. Shame I haven't discovered his add-on before I went to lengths to implement my own.
Focus On Left Tab After Closing by e10s, that I have not tested yet, but it could work in conjunction with the previous add-on.