Is it something fundamental to the different design choices, is it just which has had more test/resources thrown at it, and is the closing of the gap practical and/or on a roadmap?
- it would break all old-style extensions and users were unhappy about that;
- it takes looooots of memory and users were unhappy about that, too.
Now that Firefox has dropped all old-style extensions, there's large focus on reducing memory footprint, to allow moving to this model without costing as much memory as Chrome.
There are still a few kinks to fix, but this new isolation model should hopefully land within a few months.
Also, all sorts of security improvements are much easier now that Firefox dropped old-style extensions. Also, switching many developments to Rust makes new code much, much safer.
If FF gets rewritten in Rust, it will be a decades long effort.
So then OCSP comes along and everything will be fine now, right? Nope, Chrome doesn't (or, again, "didn't", they might have fixed this since I last looked, but that doesn't make it any better) let you do any sort strict revocation checking, so if the OCSP response failed and a response wasn't pinned, Chrome would happily let you in without bothering to check revocation (Firefox does this by default too, annoyingly, but it does have a strict mode).
Even if these things have been fixed, Chrome's general attitude towards web standards and security is to run away and not bother when they don't get their way, so even if you're not already skeptical of blanket claims that they're "more secure", which is a meaningless thing to say, there is evidence that their attitude towards web security might make them less so.