I am not affiliated with the Mozilla Dev team, but I believe this is probably not legitimate. First, Firefox is not a search engine and thus does not make money from searches (it is open source) or track them. Second, 5 billion is WAY too few searches. For instance, Google processes more than a trillion searches annually, while DuckDuckGo, which has a very low market share (sadly), processes 36 billion searches annually. So unless you use an obscure search engine, 5 billion total searches is unlikely. And the wording is indicative of a scam.

Things like that are always a scam and you should never respond or click on such a message, but close the tab if possible. See also:

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Is this a scam? I just did a search for a local pub here in Boise, and receive the following: You've made the 5-billionth search. Congratulations! You may be our next lucky winner!’ for ‘Firefox

Obviously a scam indeed.

ex: https://www.pcrisk.com/removal-guides/14415-you-ve-made-the-5-billionth-search-pop-up-scam

Probably got it from some malvertising ad on some site or you could be infected perhaps, though you are using macOS and not Windows.

Variations of the you have won a expensive current iPhone or Samsung Android phones or Tv or gift card worth about $1000 usd has been around for several years with such variations being posted about frequently for a while at times on this forum in the past. It would be a fake short survey or say your IP has been selected etc. Then they try to trick the gullible into giving out sensitive personal information like name, address, phone# and cred card for cheap shipping of only $1-3 dollars to get said so called prize.

Image below is a example of a variation being posted about at times in past.