[TheChaplain]

I installed the latest Firefox Aurora (55.08b), latest uBlock0.webext.xpi and a MITM proxy.

uBlock WebExt does not block GA.

I added a specific filter for the GA domain, but still uBlock failed to block again.

Unrelated, I noticed Firefox also made a connection to aus5.mozilla.org which sets cookies named _ga and _gid.

[gcp]

Doesn't block it generally, or doesn't block it for the "Get Add-ons" page?

The latter is expected. The former isn't. In any case you should report this to the author, not here.