I definitely plan on expanding availability to Firefox and Opera in the short-term (and eventually Edge), because porting Chrome extensions to those platforms is pretty straight forward.
Safari on the other hand is a little bit more challenging. Most browsers implement extensions as plain HTML, CSS, and Javscript — they're essentially just websites. Safari requires developers to integrate native Swift and Objective C code into extensions, which means I might have to do a partial rewrite (and learn Swift or Obj C) to target Safari.
You also have to pay a 99 dollar annual fee to maintain a presence in the Safari Extension Gallery, which is somewhat exorbitant for a free extension. If users install the Safari extension directly from my website instead of the extension gallery, then they'll miss out on auto updates.