Information in http://forums.musicbrainz.org/viewtopic.php?id=5973

From my original post:
"Maybe I'm missing something, but I can't get Picard to load the https version of musicbrainz.org when clicking "Lookup in Browser" from the toolbar or when right-clicking albums/files and selecting the same, or by clicking the album art area for found albums to go directly to that release's page. Entering port 443 throws an error. Typing in https://musicbrainz.org in the Server Address field cuts out the : making it unusable.
I found this out while attempting to add cover art yesterday, and the upload would always error out. When MB talks with CAA, CAA expects MB to be talking in https which isn't possible with "Lookup in Browser" from what I can understand. example error:
"XMLHttpRequest cannot load http://mbid-9207f6ad-7c76-4cf9-9ae7-59e ... hive.org/. The request was redirected to 'https://mbid-9207f6ad-7c76-4cf9-9ae7-59 ... chive.org/', which is disallowed for cross-origin requests that require preflight."
I fixed it on my end by always forcing musicbrainz.org to load in https via a browser setting and now am able to do "Lookup in Browser" and add cover art as I always had done. This is too much work for some people though; Picard should be smart enough to load the https version of the site in this day and age."

From a followup post from Mineo:
"Webservice.py is only used for performing HTTP requests in Picard itself, it has nothing to do with opening a webbrowser. The line that's actually responsible for the "Lookup in Browser" (LIB) being HTTP only all the time is in filelookup.py. _build_launch understands the HTTPS scheme just fine, but _lookup never sets it. If _lookup is changed to set the scheme to HTTPS, then the LIB function passes an HTTPS url to the browser (clicking the cover art doesn't do that, I'm not sure why. There's some code in webservice.py that already handles some of the "should this be HTTPS?" detection which might be useful to use in filelookup.py as well."

There's some more info in the forums, but those are the major parts of it.

Note that this affects the button found in the main bar, right-clicking files, and clicking artwork to load releases.