Since one of Firefoxes selling points for the desktop is its plethora of extensions, will those be available on iOS? If not, what's the difference with this and Safari?
They won't be available nor can they be. Firefox on iOS will be a Firefox UI over Mobile Safari, like all other browsers on iOS. Firefox's extensions are made to work with the rendering engine in proper Firefox as you'd use on Windows, Mac, Linux, and Android. Platforms where you can install your choice of browser.
IMHO, 1Password extension injects JavaScript into web page. Would be interesting if addons are possible.
Probably not, unless the UI itself is rendered in a webview, and extension scripts can be injected into the page during runtime.