Attorney General Jeff Sessions officially resigned this afternoon at about 3 pm eastern time, but it certainly appears that he did not resign of his own free will.
Sessions's resignation says he resigned at Trump's requesting; an implication that he was fired.
From the Associated Press:
WASHINGTON (AP) — Attorney General Jeff Sessions was pushed out Wednesday as the country’s chief law enforcement officer after enduring more than a year of blistering and personal attacks from President Donald Trump over his recusal from the Russia investigation.
Sessions told the president in a one-page letter that he was submitting his resignation “at your request.”
Trump announced in a tweet that he was naming Sessions’ chief of staff Matthew Whitaker, a former United States attorney from Iowa, as acting attorney general.
It's not completely clear what this really means for special prosecutor Robert Mueller's investigation, but Matthew Whitaker has been critical of the investigation in the past.
It's possible Whitaker will assert control over the investigation and may even try to limit it, but if he obstructs the investigation in any way you can bet House Democrats on multiple committees will call him to testify under oath when they control oversight.
For his part, Trump held an unhinged press conference this morning where he was visibly upset about Democrats retaking control of the House of Representatives. I believe he's panicking and I believe all he's ultimately doing is giving Democrats more reasons to investigate him.
Even if acting attorney general Matthew Whitaker or the next attorney general fires Mueller, House Democrats would have the power as the majority to rehire Mueller to serve as independent counsel.