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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.

Image of Firefox dev-tools with the description

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?

Mário Areias
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    Can you provide a public URLs for this resource I can use to replicate this and log a bug? – therealjeffg Jul 21 '15 at 06:04
  • Hi @canuckistani. Sorry for taking a long time to answer. Is there a way to send that to you in private? :) – Mário Areias Jul 24 '15 at 05:43
  • Sure, jgriffiths at mozilla dot com – therealjeffg Jul 25 '15 at 16:17
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    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](https://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. – therealjeffg Aug 05 '15 at 03:15
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    I have the same problem. Any solutions yet? – nosklo Aug 19 '15 at 12:46
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    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 – Mário Areias Aug 19 '15 at 13:00
  • @canuckistani Here is a public URL which exhibits this issue (jpg): https://upload.wikimedia.org/wikipedia/commons/b/b5/Peshmerga_on_a_T-55-Tank_outside_Kirkuk_in_Iraq..jpg – Johann Oct 23 '15 at 05:43
  • Thanks, logged [a bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1219556) – therealjeffg Oct 29 '15 at 05:14
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    FWIW this bug has been fixed and the fix will appear in Firefox 52. – Tom Tromey Feb 14 '17 at 14:59
  • The incorrect sizes appear to happen in my development JavaScript builds, but in production mode, the numbers are correct. – Dave Feb 15 '17 at 19:42

1 Answers1

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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

Mike Lyons
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