People have answered already but I want to summarize two key facts:
- Browsers are huge attack targets and if you don't update you will be vulnerable to many more vulnerabilities.
- TLS not only provides confidentiality but also integrity. This prevents others from injecting content into the sites you visit, even if you don't care if they know the content.
Mozilla used to have a page that said something along the lines of "Your version of Firefox is out of date. Please update to stay safe and secure. If you don't want to update we recommend that you use the most recent version of another browser to avoid security issues." I can't find this page anymore, and I might have dreamed it but the message is good either way.
Also if you want Firefox to get off of your lawn and stop changing so often you can try the ESR release which has feature releases far less often while still receiving security fixes.