The reality these days is websites have become simply too bloated. Almost every single darn site pointlessly bundles megabytes of javascript libraries, utilizing functionality that these browser engines were never designed to work with. In the case of TFF, it has not received any major Javascript functionality improvements in around 8 years, which means that many websites just flat out refuse to work. Engines such as UXP (Pale Moon, based on Firefox 52) and Moebius (IW55, based on Firefox 55) have constantly received Javascript additions over the years, so most websites do run, but as more complexity through Javascript is added, they're bogged down more and more. Mozilla realized that this would happen in around 2016, and Firefox Quantum was the result of them massively rewriting portions of their codebase to overcome the gradual worsening of performance they predicted would happen to the Pre-Quantum engine.