It’s a joke (from the author of the article and not a quote from Zuckerberg). Usually in the format of “xyz will continue until morale improves”, where xyz is something that is likely to reduce morale, thus implying that xyz will continue indefinitely.
In this case the morale is of those forced to consume meta’s ai slop, i.e facebook users.
An alternate worded headline would be: “AI content will continue on Facebook regardless of whether users like it”.
Based on their 2022 report, only half of their expenses were on software development costs - around $220m, and it’s not clear what portion of that was on Firefox vs other projects.
https://assets.mozilla.net/annualreport/2022/mozilla-fdn-2022-fs-final-0908.pdf
In terms of revenue: around $100m was from sources other than Google.
Therefore, it seems plausible to me that Firefox development could still be funded with $100m of annual revenue. At a smaller level no doubt, but still in existence nonetheless.