SAFE Beaker is built using electron, which is a browser engine based on chromium packaged together with the nodejs-programming-language. These only work on desktop at the moment and won't be portable.
Other ways, yes. Plugins: no. Firefox recently canned their older Plugin-Infrastructure and the new one doesn't allow for custom protocols (and there seems little hope that it will any time soon), rendering that a dead end.
However, both platforms appear to allow the usage of their own web-frames (their rendering engines) with custom URL-resolution and -fetching mechanism. We are thinking of building a minimal browsering-framework for mobile, that would allow browsing and usage of SAFE-js (-style API) and could be the base for a full-blown browser with tabs, favourites and all that fun. That is, however, a bit further down the road and we haven't fully investigated this just yet.