I can't tell you about # of firefox extensions vs chrome ones, but there's been a noticeable improvement in speed and most important of all memory usage and stability. It renders pages substantially faster and memory footprint has kept itself under 300MB at max.
Since firefox upgraded from version 5 on, a lot of times extensions cause the bloody thing to crash. This was happening on a hourly basis and firefox would also bog down with memory leaks until I was approaching 2GB of memory usage. These are known bugs that have been in firefox for several versions now and have not been addressed due to their aggressive release cycles. I got fed up with firefox always crashing so I switched a month ago. Chrome on the other hand has not had these issues in my last month
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