Firefox should be OK but, as always, it will depend on the version of FF. There can always be subtle differences but you should be OK.
Safari is theoretically only supported on iOS so you will need to check that. A lot will depend on your content and the only answer is to test it.
See www.grainge.org for RoboHelp and Authoring tips
My experience with Safari on OSX is that is works fine. I don't
regularly test it, but my clients report that it all works ok on mac.