I almost never get Firefox crashes on Linux, and I don't remember seeing significant slowdowns with text boxes either, at least not simple ones.

How long are the inputs that you get problems with?

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.

It could be a leak but it could also be an inefficient piece of logic in Firefox. One could imagine that on every keystroke Firefox is scanning the entire input text for typos or malicious inputs whereas Chrome might be scanning only the text before the cursor back until the first whitespace (since the other text is already known).