The Boomla OS is a server side platform, so it is browser independent. The Boomla Website Builder on the other hand has a browser-based editor, so there this question applies.
For visitors of your website: all browsers and platforms are supported.
For editing your website: Google Chrome and Mozilla Firefox are supported, on desktop only.
Let's rephrase it to make sure it is not misunderstood.
To edit your website, use Chrome or Firefox on a desktop computer or a notebook. This restriction does not apply to your visitors, who can use any browser on any platform, like mobile Safari on their iPhone.