I use Firefox, Internet Explorer, and an iPad.
Have no problems with either of them.
What operating system, what browser, what type of computer, what version of each? With that type of info, someone may be able to help you. On Facebook, I have to press shift plus enter to start a new paragraph. Try that and see what it does for you.
To troubleshoot something, you need to provide enough information for others to see what is happening. Statements like it doesn't work won't get the help you desire. This applies to all aspects of life and work. So a note to all people here, give adequate background, how did you do it, what did you use? Give the details of what you did and what you expected. We don't have a crystal ball that we can use to observe you!
Tom
Edit. PS. Does your enter key work in other software? Is it a hardware problem like stuck keys?