This addon works OK for vocalizing short Japanese sentences, but I found some issues, especially when trying to vocalize long texts.
Firstly, I couldn't find a way to change which VOICEVOX speaker was being used. Thankfully the default sounds pleasant, but it would still be nice to be able to change the speaker.
More importantly, the addon seems to ignore punctuation, such as the full-width comma and question mark. This makes it difficult to understand long sentences, as they are pronounced without any pauses.
I ended up not using the addon much because of the last issue, but I like the idea and hope that the addon gets improved.

正しく動作させるには拡張機能の内部ドメインをVoicevoxのAPI設定へ追加するのが必要です。

VoiceSissyを開き「⭷」ボタンで新しいタブを開き、URL内の「moz-extension://xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx」の部分をコピー

「localhost:50021/setting」を開き、「Allow Origin」に先程コピーしたドメインを設定します。

mega suspicious. i tried using it and a popup from firefox itself showed up and said there was error message. It looks cool in concept with different voices but it wasn't working.