We often need many tabs open in a browser, and horizontal tabs become unmanageable. Tab Kit allows you to have them vertically on the left, with various additional configuration choices.
I opted for the tree structure, so when I open a tab from another one it’ll show up as a child to the original. I can “lock” tabs so they cannot be closed by an accidental click or keypress. They get a “read” marker so if you open a few tabs and leave them till later you can still tell which ones you’ve actually already looked at. And there’s colour coding also.
In short, a great help. Just click the Tools/Add-Ons menu in Firefox and find Tab Kit in the extensions. Install, configure, and enjoy!