Firefox 45 has introduced a regression where the
FileReader.readAsText method raises an error when trying to re-read a file that has been modified since it was initially read. This issue doesn’t occur if the file size has not changed. Mozilla developers are working on the solution.
Update: According to the discussion in the bug, this behaviour is considered not a regression because the spec says
File objects are immutable and re-reading a modified file should always fail. To work around the issue, you should always use a new
FileReader instance to read a file.