Joining https://meet.google.com/ video calls always result in the error:
"Couldn't start the video call because of an error."
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0 ID:20190729095501
I cannot reproduce this bug. Could you provide more detail ? Could you paste your about:support page for us to have some hints? Does it work on Firefox 68 or 69?
Seems to work now on Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0 ID:20190813095543
Note for folks who happen to find this bug in a Bugzilla Search: if you see this error message in recent Nightlies (from October 2019 and later), it's likely bug 1586423.