In my experience, once people see how sidebar/vertical tabs work, they're a lot more interested. Most folks I've talked about this with have not even known there was an alternative. After someone has been shown how to change between tab layouts, they mostly stick to sidebar/vertical tabs.
Maybe I should have tried plain vertical tabs—I tried out tree-style tabs and kind of hated it, so I went back to horizontal.
Regardless, the question isn't really whether vertical tabs would be a useful feature, it's whether it would be the most useful thing Mozilla could be working on.
I'd much rather see them pour resources into making Firefox's devtools the best again. I'm pretty much the lone pure-Firefox holdout among my coworkers, with everyone else at least switching into Chrome for serious devtools work, and most now run it all the time. Make Firefox the best for devtools and you ensure it has reliable support across all browsers and put it back in the hands of the biggest browser-evangelists.
