I find this a wonderful improvement over putting the tabs along the top, partly because it's a better use of space (vertical space is more in demand, and websites don't take advantage of extra horizontal space) but mostly because I can see all my tabs at once. This is with the tree style tabs addon, which also groups your tabs automatically as you open them.

In the normal firefox view, however, not all your tabs fit on the same screen:

The tab listing dropdown on the right is just the popup you get if you click the down arrow to the right of all the tabs

In firefox if you have too many top tabs you have to scroll. In chrome it shrinks them down to favicons, and then you have to scroll. Finding a tab is very slow when you can't see the title.

Chrome doesn't support side tabs [1], and this has brought me back to firefox after six months on chrome: I wanted vertical tabs back.

[1] They did experiment with them, and decided they didn't pan out. You used to be able to enable a hackish semifunctional version with a command line option, but that's been removed. There is an extension called tree style tabs (beta) but it is unrelated (and not useful).