Image Replacer от f3rufus

This extension allows you to replace images on a web page with your own via URL or directly from your device.

HOW IT WORKS

Desktop
Right-click any image → select "🔄 Replace this image" from the context menu → choose a replacement URL or upload a local file → click Apply.

Mobile (Firefox for Android)
Long-press any image (hold for ~0.3s) → the replacement panel slides up from the bottom → choose URL or file → tap Apply.

The replacement is applied immediately and saved. Next time you visit the same page, the image is swapped automatically.

IF SOMETHING GOES WRONG

Some web pages can replace URLs after certain actions, such as in chats. The extension performs the replacement based on the URL. If the replaced images have disappeared and the "This Page" section is empty, try applying the rules again; the address may have simply changed.

Also, if you're replacing an image with another via URL, make sure it's still accessible via your link.