|>>|| No. 23064
Firefox just don't have the development resources. They're a small and relatively underfunded open source project, competing against huge multinationals.
They have the oldest layout engine of any modern browser (Gecko, dating back to early 1997), which means dealing with a lot of legacy code. Maintaining old code is immeasurably harder than writing it from scratch. Gecko is also burdened by a lot of early engineering decisions that bloated the scope to be much more than 'just' a browser layout engine, which makes the architecture hugely overcomplicated.