The encoding br is not supported.
Fiddler should decode response encoded with br (Brotli) so that I would be able to read them.
Fiddler displays the raw encoded response which a human cannot read.
Update: The Brotli encoding is now fully supported in Fiddler Everywhere.
With the latest version (1.5.1) response body will be automatically decoded in all inspectors except for the Raw inspector, where we have a manual decode option (this provides an opportunity to see the original response vs. the decoded one)