The Firefox developer version¹ has a full file menu on its code viewer, with save, open, refresh, etc.
I believe you can activate it on the main build too, but I don't know how. Anyway, if you are debugging code, I highly recommend the developer version.
1 - That is a different configuration of the same code as the main build, for web developers. It's not for developing Firefox itself.
Also, how can I edit code? It is non editable for me.