You can mitigate the security concerns through other sandboxing techniques (e.g. Qubes, VMs). It's not like up-to-date Firefox is really that secure either.
Web developers stand the most to lose from people opting to run old browsers. Back in the IE6 days, it was justified, but by now, web APIs are already about as powerful as they're going to get, and we're reaching (or are long past) the point of decadence, churn, and bloat.
If you want to run old Firefox, take reasonable precautions, and you'll be fine.
Edit: down to -4. FWIW you can also securely run IE6 or literal malware.
Web developers stand the most to lose from people opting to run old browsers. Back in the IE6 days, it was justified, but by now, web APIs are already about as powerful as they're going to get, and we're reaching (or are long past) the point of decadence, churn, and bloat.
If you want to run old Firefox, take reasonable precautions, and you'll be fine.
Edit: down to -4. FWIW you can also securely run IE6 or literal malware.