Unable to download on Firefox ESR 68 due to HTTP 401

[JCGoran]

Recently, I haven't been able to use Send on Firefox 68 ESR, due to a strange bug: basically, every time I try to access an uploaded file using Firefox, the download button does not appear at all. If and look at the error console, I find the following a whole bunch of times:

GET https://send.firefox.com/api/metadata/fca5b7a88ed898c5  401 Unauthorized

The following screenshot captures the behavior:

Note that after leaving the tab open for some time, I get 429 Too Many Requests when trying to access https://send.firefox.com (guessing due to rate limiting?).
At first I thought one of my addons or about:config was responsible, but I can reproduce it on a completely fresh profile. It works just fine on Chromium (v 82).

Platform: Debian Unstable
Browser: Firefox ESR 68.8.0