You can try to use the Live Http Headers extension to see if that catches those requests.

• https://addons.mozilla.org/firefox/addon/live-http-headers/

Live HTTP Headers (and Firebug) had no problem in showing the network info, so the issue was with Firefox Developer Tools' Network tab alone.

The good thing is that it is fixed now, probably after I updated Firefox to the latest version. To anyone else experiencing the same issue, update your browser and that should fix it.

Edit: Fixed typo. Added comment for others with the same problem.

I have the latest version but the problem still existed. After several failed attempts, I finally solved it by editing the settings in about:config. To do this, I set a new value for the parameter devtools.netmonitor.filters equal to ["all"] In my case, there were two parameter values. In this case, the name of the second value could not be read, because there was a strange sequence of characters. Thus it was impossible to select the filter tab in the interface.

If some pref(s) were corrupted then you may have to check if that is the only one and possibly check the prefs.js file or delete this file to reset all the prefs.

• http://kb.mozillazine.org/Preferences_not_saved

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)