Yeah flash is 32 bit but flash has been dead for the greater part of that decade. Not sure how it could possibly be the case that moz was still shipping 32-bit Firefox as default until yesterday.
Consequently, when you're shipping software for Windows, it's beneficial to have a 32-bit version to capture that part of the userbase. But since 64-bit Windows will also happily run 32-bit apps, once you have a 32-bit version, there's no particular reason to have a 64-bit one. The only time you get something from 64 bits is when you need a lot of memory, which most apps do not. On the other hand, by having a single version, you simplify the acquisition and installation story for the users, cut your testing matrix in half etc.
Hence, most Windows software is still 32-bit.
These are the realities of shipping for the minority not the majority. Users who are a blip for some companies end up being huge to others.
https://hardware.metrics.mozilla.com/#goto-os-and-architectu...
Example: https://blogs.msdn.microsoft.com/ricom/2016/01/11/a-little-6...
But I admit, these are the exception to the rule.
Having used both Linux and Windows in 32 and 64 bit versions - in a few cases on the same machine -, I did not notice a difference in performance[1]. If the performance hit due to 64bit is really that substantial, I could imagine the larger number of registers (and possibly larger caches) make up for that.
[1] Possibly, as so often in the days before SSDs, I/O was enough of a bottleneck that the difference between 32 and 64 bit code became unnoticeable.
And saying the flash has been dead for the greater part of a decade is just completely wrong. You have to be just really, completely out of the loop if you think that applies to any kind of reality for general users.