Failure to load subtitle link favicon using /proxy?url

[punamdahiya]

[Steps to reproduce]:
- Save a shot
- Paste the saved shot's link in Chrome.
- Console shows message Failed to load resource: the server responded with a status of 404 (Not Found)

[punamdahiya]

[punamdahiya]

Issue reported in #4710 could be fall out of this error in Google Chrome
#4710 (comment)
Also, This call seems to be redundant as favicon for subtitle link is hidden #4710 (comment)

[punamdahiya]

LInks to see issue in dev,stage and prod
https://screenshots.dev.mozaws.net/HxOj9GaxPyZc8ity/www.facebook.com
https://screenshots.stage.mozaws.net/hnjHvFJIrNdLE3yW/www.theatlantic.com
https://screenshots.firefox.com/97dlRupgFyXtHvxX/www.facebook.com

[jaredhirsch]

Looks like createProxyUrl (in proxy-url.js, called when creating the shot page) assumes a regular https URL for a favicon, while we are saving the favicon as a data-URI.

Indeed, if you look at the data submitted in the PUT request when creating a new shot, the favicon is indeed saved as a base64 image, rather than, say, https://facebook.com/favicon.ico:

[jaredhirsch]

When the shot is created in the background page, we get the favicon URL from Tabs.tab.favIconUrl:

https://github.com/mozilla-services/screenshots/blob/master/addon/webextension/background/takeshot.js#L13

Apparently this API now returns a data-uri instead of the URL of the original favicon.

[jaredhirsch]

Ah yeah, it turns out that the addons code just gets the favicon from tabbrowser (deeper code in Firefox), and some favicon refactoring there resulted in the image itself, not its URL, being returned by tabs.Tab.favIconUrl.

This is definitely an upstream bug; I'll update this issue when I get a BMO link for the Firefox bug to fix it.

In the meantime, we should just stop loading the favicon on the server pages altogether.