Question: Do I have to tell the firm with whom I am interviewing my current salary?
You don't have to, but it is recommended to do so if asked, for the obvious reason that you don't want to look like you're hiding something.
Many candidates are hesitant to reveal their current salary because there can be many factors that affect one's salary that are not apparent when simply providing someone a number. Some of these factors may be personal and/or not relevant to the candidate's prospective position, thus making the candidate not want to reveal them. For example, a candidate may have taken time off for a medical issue that has been resolved. Or, a candidate may happen to work for a firm that happens to pay out of scale for the market (higher or lower).
Also, revealing one's current salary is similar to many other things in that going first can put you at a disadvantage. Candidates may feel that they will get a lowball offer because they are currently making less than the prospective firm expected. Or, candidates may worry that the firm will not consider making them an offer because their current salary is higher than the prospective firm wants to pay for the particular opening.
If you are intent on trying to avoid revealing your salary, you could tell the firm with whom you are interviewing that you have knowledge of their general salary scale and that you are convinced that salary will not be an issue. Or, you can give a ballpark figure, which they may accept. For example, you could say that your base salary is in the high- five figures, or that, including your bonus, your salary is in the low-to-mid-six figures. Or, you could assert that your current salary is for your current job, from which you are interested in moving on, and it should not be an important factor in determining your fit for a new position. These tactics, however, may not get you very far.
There are many legitimate reasons why a firm with whom you are interviewing wants to know your current salary. First, it (in theory) gives the firm an idea of your salary expectations. Second, it (in theory) gives the firm an idea of where your current firm places you with regard to class year, work product, etc. And, yes, third, it (in theory) gives the firm a ballpark figure of what they need to offer you for the position in which you are interviewing.
Whatever the reason you may be hesitant to tell a firm with whom you are interviewing your current salary, if you are seeking or are willing to accept a change from your current salary, get over your fear and be up front with the firm about your reasons. If you are looking to make a lateral move to make more money, then you must be able to accept the work and responsibility that likely accompanies that, and the firm should know that you are prepared for, and/or possibly specifically seeking that. It is also important for the prospective firm to know that you are willing to take a cut in salary because you want to move to a particular geographic location, change areas of law, or want to take a step back from your current workload. The best way to make sure that everyone is on the same page is to be open about your wants and needs, which will help ensure that any match is a good fit.