Why transferred bytes are bigger than size bytes in firefox dev tools?

Question

I am measuring the performance from a website. When looking at the firefox-developer-tools, I noticed a weird behavior. There is a specific JS file which the transferred size is 2,831.54 KB, but the actual size is 1280kb.

According to Mozilla, the Transferred size should be smaller or equals to Size:

Transferred (new in Firefox 38): the number of bytes that were actually transferred to load the resource. This will be less than Size if the resource was compressed.

Size: the size of the resource, after any decompression.

Firefox Developer Edition version: 41.0a2 (2015-07-19)

What could have caused this behavior?

Gothenburg

Can you provide a public URLs for this resource I can use to replicate this and log a bug? – Lewiss

Hi @canuckistani. Sorry for taking a long time to answer. Is there a way to send that to you in private? :) – Dioecious

Sure, jgriffiths at mozilla dot com – Lewiss

As a follow-up, Mario gave me a url to test and I saw an odd effect - a specific file where the transferred size was much larger than the actual size. When I used wget or curl I got the same size in the content length header. Chrome reported the same size. Then I copied the file off of of the server to a different server hosted by surge.sh and saw completely different ( and much more normal ) transferred / real sizes reported by all tools used. I suspect this is a server configuration / http quirk. – Lewiss

I have the same problem. Any solutions yet? – Helsinki

Not really. I haven't investigated further, but it seems something wrong either in the server or in Firefox itself. When using wget or curl the result is displayed correctly, so we don't know – Dioecious

@canuckistani Here is a public URL which exhibits this issue (jpg): upload.wikimedia.org/wikipedia/commons/b/b5/… – Nord

Thanks, logged a bug – Lewiss

FWIW this bug has been fixed and the fix will appear in Firefox 52. – Benedikt

The incorrect sizes appear to happen in my development JavaScript builds, but in production mode, the numbers are correct. – Husch

Answer 1

As stated in the most upvoted comment on the question:

This appears to be a rare bug in Firefox that has been fixed in Firefox 52:

FWIW this bug has been fixed and the fix will appear in Firefox 52.

– Tom Tromey Feb 14 '17 at 14:59

Disembody

As of 90.0.2, I can still see transferred > size – Ingather

Mozilla Firefox 107.0. Still the same bug. – Jemadar