Solution: delete all bookmarks that point to an article hosted at github.blog.
Background: For the longest time, Firefox would suggest the github.blog web address whenever I type "github" into the address bar. I found that weird: yes the word "blog" starts with a letter lower in the alphabet than the word "com", but the ".com" TLD is much more popular so should show up first, right?
Right... unless you, like me, have web search suggestions turned off when entering web address into the address bar. Instead, it takes suggestions from my bookmarks and open tabs, like I instructed it.
Thus, Firefox is behaving exactly as designed and instructed, and the solution is to remove the bookmarks that point to github.blog.
I only wish I'd had recognized that sooner...
Thank you for your service.
Now if only Firefox mobile stopped autoplaying videos when I told it not to autoplay videos.
That indeed is annoying.