Addons.mozilla.org is returning HTTP 503 – Backend.max_conn reached when accessing the Firefox Add-ons application

Add-ons addons.mozilla.org
1

Problem

addons.mozilla.org is returning HTTP 503 – Backend.max_conn reached when accessing the Firefox Add-ons application.

Technical details

On 20 August 2026 at approximately 17:52 BST, from the UK:

https://addons.mozilla.org/
→ HTTP/2 301
→ /en-US/firefox/

https://addons.mozilla.org/en-US/firefox/
→ HTTP/2 503
→ Backend.max_conn reached
→ Error 54113
→ Server: Varnish

The root domain is reachable, but the actual Add-ons application endpoint fails.

The response headers show the request passing through the London Fastly edge:

x-served-by: cache-bfi-kbfi7400086-BFI,
             cache-bfi-kbfi7400086-BFI,
             cache-bfi-kbfi7400089-BFI,
             cache-lon4278-LON

x-cache: MISS, MISS, MISS
retry-after: 0

An earlier successful request to / also showed:

x-served-by: ... cache-lcy-egml8630080-LCY

The original browser error included:

Error 503 Backend.max_conn reached
Error 54113

cache-bfi-krnt7300023-BFI

A successful response to / provided this AMO request ID:

e8394a86-2caf-4690-b13c-057ec5fb3496

This appears to be a backend/CDN capacity or connectivity problem affecting the AMO application rather than a DNS or local browser problem.

Please investigate the AMO backend capacity and Fastly/Varnish routing for the UK/London edge.

Thanks.

2

Looks resolved for now, at least I was able to submit my addon ~30 minutes ago

3

Yes - it was fixed pretty quickly - thanks guys.

4

Hi @plektis and thanks for reporting!

Apparently, this was part of a larger issue that affected many of our services, but AMO took the biggest hit.

As you have already noticed, the issue has now been resolved. Thanks again for your detailed report!

-Christos

5

Hi Christos,

You’re very welcome and thanks for fixing it so quickly. I can continue now with making some screencasts with audio showing how to install and use TubeStopper: