I was developing a new site, and could not find an easy way to perform image fallback testing in Firefox Developer Edition - Developer Tools.
In about:config, there is an option to disable avif, which works for that format. There is no option for webp.
In the Network tab, there is a request blocking feature. But, it isn’t clear what to use to block a specific image format. If I enter avif or *.avif as a new string in request blocking, this completely breaks the images on the site.
I found a solution in Chrome Dev Tools. Using Ctrl+Shift+P opens a command palette, and there are options that exist to disable image formats. Specifically, Disable AVIF Format and Disable WebP format. Using these options, I was able to successful test the image fallback support for my site.
It seems like this feature should be an option in developer tools for Firefox.