For instance neither "classic" top tabs nor a side list/tree is the "best" or "right" way to design a tab selector. Some users will prefer one, others the other. You should give them choice.
I think building support for a choice about something like tab placement is a waste of time and code - I don't understand why people can't adapt to one way or the other and then we all benefit from simpler code with fewer bugs.
I don't think we need to decide if top or side tabs are better, because I can't see how it matters much. Just pick one, hard code it, and everyone get on with the things that matter.
I spend several hours every day on a browser, switching tabs hundreds of times. I am measurably and subjectively more efficient with side tabs than top tabs. Why is this something that doesn't matter? It's not trivial, I literally cannot browse without side tabs without getting annoyed. It's absolutely not a waste of time and code if it's important to people.
Not really a "configurable option" but very close to it, it achieves the same goal and has been released along with the Backspace modification.
Just customize!
Not supporting images or JavaScript would simplify things as well...
That's just your opinion—which I guess the Firefox devs and most users don't share.
Customizability is a very useful feature for many people.