Firefox throws XML parsing error

Firefox apparently doesn't know how to handle plaintext responses, and instead expects XML from the server. Chrome is fine. Here is the error message:

XML Parsing Error: syntax error
Location: http://ipenthusiast-env.eba-x28ab2gb.eu-north-1.elasticbeanstalk.com/Task1_Servlet
Line Number 1, Column 1: Task1_Servlet:1:1

There is a relevant stackoverflow question with two possible remedies. I tried overriding the request mimetype in AJAXlib.js, but it didn't work. Here is a link to the discussion: https://stackoverflow.com/questions/51000009/i-keep-getting-this-error-xml-parsing-error-syntax-error-but-still-the-website