Firefox addons on iOS, ported directly from the desktop versions with no modifications, will be possible. Gecko has a lot of under-the-hood knobs and dials that simply don't exist in Webkit, but are needed by the addon ecosystem.
Yes, you can run standard desktop Firefox extensions on FF for Android. There was a whitelist of extensions Mozilla allowed you to install though, I'm not sure if that still exists on the most recent versions.
The hoops are far too technical, finicky, and poorly documented for the average user to curate their own add-on list for Fx. It's a real shame since I think a lot of it is to make sure the add-on's UI is responsive, but many of most used add-ons on desktop have no UI to speak of (like redirecting AMP links, skipping short links, etc.).
You can use uBlock Origin on Firefox for Android. The biggest feature I miss when I switched to iOS, although I've found Orion beta also has this feature and is decent.
