In Firefox 39 and 40, no matter where in a table cell I click, my cursor is placed at the very end of the cell content. If try to use the arrow keys to move the cursor into the content of a one-line cell, nothing happens; if I try to do same thing in a multiline cell, each press of the arrow moves the cursor up one line. This makes it extremely difficult to edit table cells because almost all cursor positions aren't actually reachable.
Things work correctly in Firefox 38. Firefox 39 is currently in beta and goes to general release the week of 23 June 2015.