I'm quite confident to say that millions of people use Firefox to comment on Reddit or similar sites every day, or write long posts, without seeing this problem.

Without knowing more about your configuration, it's hard to give advice, but definitely worth trying with a clean profile first.

If you don't report this problem upstream it will never get fixed, as obviously no-one else is seeing this. Firefox has a built-in profiler that you can use to report performance problems like this.