Since the launch of collections last year, one of the most common feature requests AMO gets is the ability to install all or some of the add-ons in a collection at the same time. There’s really only one thing that has held us back from offering this functionality, but unfortunately it’s not something easily overcome: conflicting first-run experiences.
These days, almost every add-on has some sort of first-run experience, whether it’s a new tab that’s opened, a sidebar, a wizard, or (worst of all) a modal dialog. When several add-ons are installed at the same time, these elements all fight for attention, often in confusing and unexpected ways. I wrote a post on this some time ago that showed an example of what havoc can be wreaked with only 3-4 add-ons, as well as some suggestions on how developers can improve this area.