As of March 13, 2026, this issue seems to have been resolved.

This is a demonstration of how recent changes to GitHub Pages makes it impossible to use HTTP Ranges with Firefox.

The page hosted at bdon.github.io/ghpages-firefox-range-bug/ works correctly on Chrome and Safari, but will throw a 416 error on Firefox.

The file object_short is the text Hello World! compressed with gzip.

The page at index.html fetches all bytes using a range request Range: bytes=0-38 and then uses the DecompressionStream API to get the original contents.

The file object_long is the same content as object_short but with 5 megabytes (1024 * 1024 * 5, or 5242880 bytes) of zeroes prepended:

( dd if=/dev/zero bs=1M count=5 && cat object_short ) > object_long

When we instead request the same byte range at a 5MB offset, bytes=5242880-5242918, the GitHub pages server returns a 416 Range Not Satisfiable error.

If you look at Firefox's request headers, it is the only one to send the Accept-Encoding gzip, deflate, br, zstd, identity header in the request. Other browsers send only identity.

In GitHub's CDN configuration, responses are being force-compressed if certain conditions are met:

1. The request includes the Accept-Encoding: gzip header
2. The resource is over a certain size (5MB?)

You can see this because the 416 response does return a content-range bytes */5183 header, indicating the CDN thinks the resource is about 5kB, which is similar to the result of gzip object_long.

Renaming .pmtiles to .gz or .pmtiles.gz should disable server-side compression completely.

A PR is open on the mime-db project to mark .pmtiles as uncompressible.