This works great, but I have some observations/suggestions:
1) When I open all in new tabs, it also opens the home page (site URL) of the feed. Firefox's Live Bookmarks did not have this behaviour.
2) Under More Options, I just discovered today that I can set a custom symbol/emoji for the Unread and Read Status of a bookmark.
This feature should be highlighted in your description and in the changelog.
3) The default for the Read status is this, ✓, a black tick mark, which makes it quite hard to see at a glance.
I changed it to ✅ which I think looks much better. I think you should make it the default or at least suggest it as an option in the description of this addon.
4) When I changed the symbol for Read status, I had to Reload Live Bookmark, or restart Firefox for it to apply to all 10 feeds I have saved in a folder. Please apply the change instantly.
Thanks for this great addon!

Edit:
It would be very helpful if you could add an option called Open All Unread in Tabs, above Open All in Tabs!
Right now I have to Open All in Tabs, remember the oldest unread tab name, find it, then Close Tabs to the Right to be left with the newest blog posts I have not opened before.