Complain to Mozilla, it's their bug (lots on google about it). There are also notes on some changed options in the upgrade guide. Your feed might not have auto update enabled. They are also removing the RSS reader in the next version. You can also try a rss plugin instead of stock.
Starting in Thunderbird 60, each feed's update interval may be individually set. If the checkbox Check for new articles is unchecked, the feed is 'paused'. If all feeds in a folder are paused, the folder is paused and will appear in lower opacity. A folder may also be paused/unpaused using the context menu -- the status will apply to all feeds in the folder. The entire account may also be paused (this does not change any individual feed's pause state).
I've copied the Laracasts file, changing the rel self tag to my own site, uploaded the edited XML file to my own server and added that to Thunderbird. The file validated at https://validator.w3.org/feed/
It worked fine.
The only difference to my file and the on on Laracasts is the removal of the white space before the opening Xml tag (i.e. before <?xml version="1.0" encoding="utf-8"?>) and the removal of of the new line after <id>tag:laracasts.com,2018:/feed</id>.
I wouldn't class it as a Thunderbird issue when you can remove some white space so the file validates on the Laracasts end.
