There has been extensive research to demonstrate that it is very easy to bypass by using bots (which, is, all the purpose, right?) and only serves as tracking and advertisement purpose these days.
There are even plenty of plugins that can bypass it automatically in Firefox’s own extension market.
If bot were really a concern, there are still a lot of more effective and more privacy concerning (why we use firefox?) anti-bot captcha services, such as cloudflare’s.