If the signature file is actually a directory, upon attempting to start a new message, mozilla will hang, and proceed to eat 100% processor time until it is killed. I have no idea how/why one of my users had a ~/.signature directory. Any ideas on that one? :) I'm using debian(sid)'s compile of mozilla-0.9.4-2 and mozilla-mailnews-0.9.4-2.
This bug should be fixed now (17th) According to comment in bug 97585 it may have been mozilla itself creating that directory at one point: bug 66668 *** This bug has been marked as a duplicate of 97585 ***