My first thought was that I must have accidentally visited a website or downloaded something that exploited the browser to insert advertising shortcuts. However, I discovered that this is actually a built-in Firefox feature called 'Sponsored Shortcuts.'
Mozilla could have been an exemplary company, but they continue to squander the trust people place in them. It's shameful.
PS: You can turn off sponsored shortcuts here https://support.mozilla.org/en-US/kb/sponsor-privacy