Do you have a citation supporting that? The old engine compares quite favorably with Chrome and Mobile Safari.
https://caniuse.com/#compare=firefox+79,chrome+84,safari+13....
The provided links only compare API coverage, while other domains (performance, security...) are important. Even so, Firefox is clearly trailing Chrome, its main desktop competitor. It's not surprising since I've heard that the latter has much more man-power. And the gap will increase.
I've looked at CanIUse, ignoring the features which are implemented but disabled by default. Chromes lacks 5 features the Firefox has. The biggest two are MathML and VirtualReality, which were both available as experimental features in past Chrome versions but were removed for lack of interest in publishing or maintaining them.
On the other side, Firefox lacks 25-30 features that Chrome has. A few of them are minor, but most of them seem complex.
"our source told us Moz will likely pocket $400m to $450m a year between now and 2023 from the arrangement, citing internal discussions held earlier this year."
[1]https://www.theregister.com/2020/08/14/mozilla_google_search...
What the hell are they doing with the rest of the money? It should be "paid back" to the shareholder (the foundation) as tax free dividends that are kept in a fund for the purposes of that foundation.
If they don't invest in a new engine, and given that the old engine is not much to compare with Blink or even Webkit, what's left for them? Pocket and some BS VPN service?