If a handful of developers at SerenityOS can write a web browser (including Javascript and CSS support) from scratch in a year or so why can't Mozilla rewrite the FireFox engine in Rust?
Most definitely! But how many bugs were in the code that are no longer there? And how does the relative severity compare between new bugs and old bugs?
When considering a rewrite, the question isn't whether there are latent bugs, the question is whether fixing those bugs is worth the regressions that will likely be introduced (not to mention the cost).
Every project should have exactly zero (0) memory bugs. That's nigh impossible to achieve with large codebases written in C/C++ or some other unsafe language.
Chromium and FireFox are still busily fixing memory bugs twenty years after the code was written. It's a God-awful mess!