I'm unsure what you're referring to. I use Firefox Nightly, but your prior comment implies the existence of a "dev mode which is a separate install that doesn't update"; Firefox Nightly updates just fine, and it's not a separate install, it's the only Firefox on my machine. It's perfectly stable, I haven't had a crash in years. And I didn't need to change any settings to install my add-on, I just clicked the "install from disk" option in the add-ons menu.