My attempts to place a cursor then move it in a field usually fail.
  Try these cursor movements on Mozilla’s own Firefox support pages.
   Oddly, the text entry field for this comment is better behaved, Still it lacks many of the cursor movements I mentioned here.

Editing text in data entry fields is difficult when using Firefox for iOS. Cut and paste are often unavailable. There is no way to move the cursor in any direction other than backspace. Delete should be available as well as nondestructive cursor movement. My attempts to place a cursor then move it in a field usually fail. Try these cursor movements on Mozilla’s own Firefox support pages. Oddly, the text entry field for this comment is better behaved, Still it lacks many of the cursor movements I mentioned here.