The firm in question is “Fusion GPS” and the dossier is the infamous, ridiculous and discredited “Russian Hookers and Pee” document used to libel the President. The co-founder, Glenn Simpson refused to testify and said that even if he did show up, he’d just plead the 5th.
There are myriad connections between Fusion GPS and both members of Hillary Clinton’s inner circle as well as CrowdStrike, the tech firm that to this day is still the only entity to be granted access to the DNC’s allegedly Russian hacked server.
H.A. Goodman did a good, short video on these entities a few days ago in regards to their unwillingness to back up their claims and appear for committee hearings: