All addons on AMO that are not part of the recommended programm have that banner. Mozilla simply doesnt have the resources to manually monitor all addon, so they per default mark all addons as potentially unsafe. If you search around on https://addons.mozilla.org and you'll see that very few are without that notice.
In general if an addon already exists for a longer time and has some users, it is likely that it is rather safe, since otherwise people (developers or mozilla employes) are likely to notice any malicious activity, and reported or removed that addon.