Having your tabs arranged vertically instead of horizontally in your browser seems to be quite unusual, as it seems. Chrome doesn’t support moving your tab bar to the left or right, multiple extensions exist for Firefox. Over the years, several Firefox extensions were made, got abandoned and became outdated. Others, TreeStyleTabs, are quite complex for the simple task I wanted to solve. Luckily, somebody revived VerticalTabs in a Github repository and keeps the code updated. So if you only want vertical tabs and nothing more in Firefox, get it. :-)
Thank you, Darrin!