Firefox doesn't support the content property in the same way as Chrome — on img elements and/or when the source is an image.
<img> represents a content image. If you use it, it should have a src and an alt.
From the specification:
The src attribute must be present, and must contain a valid non-empty URL potentially surrounded by spaces referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.
