"Sr Trump,the intelligence report is devastating. Losing election by more than 3M votes and in addition this.Are you a legitimate president?" Fox tweeted Friday night.
Fox's jab at Trump comes after the US intelligence community concluded in a declassified report released that Russian President Vladimir Putin ordered an "influence campaign" aimed at hurting Hillary Clinton and helping Trump.
In addition to that, Trump lost the popular vote to Clinton by 2.9 million votes -- not more than 3 million votes, as Fox said in his tweet -- which has been something of a touchy subject for Trump, who has cited his Electoral College lead to highlight his win.
Fox also slammed Trump after he said that he is planning to have Mexico reimburse US taxpayers for his campaign promise to build a border wall between the US and Mexico.
Fox tweeted, "TRUMP, when will you understand that I am not paying for that f------ wall. Be clear with US tax payers. They will pay for it."
And on Thursday, Fox tweeted, "Trump may ask whoever he wants, but still neither myself nor Mexico are going to pay for his racist monument. Another promise he can't keep."
Trump is still insisting
that Mexico will ultimately pay billions for the construction of a massive wall along the southern border.
Fox was not the only former Mexican president tweeting at Trump on Friday.
Felipe Calderon tweeted, "To @realDonaldTrump : The more jobs you destroy in México, the more immigrants the American people will have. Think a little!"
Trump's transition team did not immediately respond to CNN for a request to comment on the tweets.
Fox was one of Trump's harshest critics during the 2016 election. During Trump's campaign, he called him a "false prophet" and said the then-Republican nominee heralded the return "to the era of the ugly American" during an interview in May.
And most famously, Fox cursed on live television in February while talking about Trump and his proposed border wall, telling Fox Business, "I'm not going to pay for that f---ing wall."