I have noticed slowdown in Firefox's UI on both machines. More important than number of tabs, is CPU usage. For example my HTML5 heavy trading platform often causes the single-threaded Firefox UI to freeze and slowdown on both machines, while I have never noticed Chrome freezing when this site is open.
On the other hand, Chrome's UI runs smooth as butter until either open tabs or other programs bring memory usage to over 90% Physical Memory in the task manger. Recent builds hit that wall a lot quicker than it did a year or so ago.
As soon as you have some heavy JS usage website, even 50 Tabs can slow down the UI. This is on Quad Core Ivy and 8GB Ram with SSD.
The amount of JS on one website now is getting insane. Chrome have similar problem as well like the OP have said. Just a different one.
So it is not FUD at all.
So when you say "some heavy JS usage" what exactly do you mean? Certainly not stuff like google docs/mail/calendar, and they're all heavy on the JS...
Alternatively, since you seem to be a power user, you could consider upgrading your hardware to 8 or 16 GB of memory; it's not that expensive nowadays, and given your power user status, more memory = faster computer experience = higher productivity. Or just more tabs.
[old man mode] Back in the day we upgraded our computers instead of blaming software.