What does your retainer letter say? Are you a client that paid the attorney fees and disbursements and provided a retainer, or a contingency client who has paid nothing but where the attorney gets some percentage of a recovery. What reasons did the attorney give for firing you in the letter? When you say "tough questions", do you mean you were curious about the lawyer's strategy or that you had a big difference of opinion in major matters, like taking a settlement rather than going to trial? Is this lien to recover money you would owe him under your retainer agreement for out of pocket cost or work he's devoted to your case, or unpaid bills? All of these factors make a difference in how one would answer the question you've posed.
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If he fired you without good cause, the attorney would be entitled to costs, but not to a fee.
If you fired him he is entitled to costs and a reasonable fee for services rendered.
If work on the case was substantially completed he is entitled to costs and the full fee - as long as the fee is reasonable per the factors in ethics rules. These include the attorney's reputation and experience, the difficulty/novelty of the issues, what other attorney charges for similar services, etc.
This answer is based on limited information provided and is not a substitute for legal advise. No attorney-client relationship is created. To protect your interests it is best to seek a private consultation with an attorney.