When more and more parts of FF are written in rust, I thought FF will get a lot faster. But for the past 1 year, I can only see marginal improvements w.r.t speed. And occasionally FF hangs when I have 20 tabs opened.
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That's really suprising to read when on my low-end laptop from 2012, Firefox runs with 20 open tabs without ever facing any issue (I have something like 30 tabs open right now).
At Mozilla's level, though, I wouldn't necessarily expect that to hold water. They should be able to write great code in either language.
A rewrite (by people familiar with the original and its pain points) is an excellent opportunity to optimize.
I vaguely remember reading here about a company that slapped a backend together in Python. When they took off, the compute costs stung, so they rewrote in Go. Saved most of the bill. Some time later that started creaking, so they rewrote again. In Python, but managed to save most of the bill again.