In the attorney-client relationship, you are the employer and the attorney is the employee. Therefore, yes, absolutely, you may terminate his services or "fire" him, as you put it. However, you cannot hire an employee and terminate him without paying him for his services. This is true of an attorney, just as it is with any other employee. If the attorney is on a contingency fee arrangement and is terminated, in most jurisdictions, he may tabulate his hours, present a bill, and place a lien on the prospective recovery for services rendered. Thus, even if you terminate him, you will still have to pay for his time. Your question does not indicate why you wish to terminate him; in fact, you need no reason, but, you should examine why you wish to do this. If you had an disagreement, try to see if you can find common ground. If he disappointed you in some way, arrange a meeting and see if you can get an adequate explanation, especially if he has been on the case for a long time. You may lose valuable time, and, possibly, money, by trying to integrate a new attorney into a very mature case. You owe to yourself to pause, take a breath, and make sure you are making a sound decision in terminating an attorney.
Yes, for any reason at any time. The lien issue is a little murkier. The Nevada supreme court seems to favor the position that an attorney most prove the value of his/her services before being able to enforce a lien, but it is not entirely clear. Sometimes the lien language is enforced as written.
Yes. Depending on your contract you with him, you will probably owe him for his/her time, most likely based upon his hourly rate if he/she claims an attorney liein on your case.
Also just because you might fire your attorney that might have referred you to certain doctors and signed liens w/those doctors, doesn't mean that you don't have to see that any new attorney pays off those liens.
You should be happy with your lawyer, if you aren't , let him know, specifically what concerns that you have. If you can't resolve them, together, then find a lawyer that you are happy with.