The "many tabs" situation where Firefox performs better than Chrome is more like 50+ or 100+ tabs than 20. People have widely different use cases with tabs.
My experience is that performance on Chrome degrades beyond 30 or 40 tabs, though the exact value depends on your system.
You can have Firefox open with 400 tabs without too much performance degradation. Many tabs will be swapped to disk or not even loaded, but the tabs you actually use will work just fine.
(also, Firefox on Mac seems to perform less smoothly than on other platforms, though I'm not entirely sure why)
> The "many tabs" situation where Firefox performs better than Chrome is more like 50+ or 100+ tabs than 20.
For me it's much sooner... on my work machine with limited memory, firefox is almost always noticeably better after 3-4 tabs, or even fewer when they're memory-hungry sites like gmail. With only 1GB on that machine, I tend to be hyper-sensitive to memory usage, and unfortunately chrome tends to basically use up all memory on the system once I reach 8-9 tabs. FF gets a fair bit further.
The upside of chrome on that machine, however, is that the process-per-tab thing makes much easier to control memory usage: although tabs chew up memory quickly, closing a tab gives you back all the memory it was using, whereas with FF the relationship between closing tabs and giving memory back to the system is much fuzzier....
As Wilya says, 20 isn't much. A single branch of my tree-style tabs is saying that it has 25 child tabs; and I have ~30 top-level branches (admittedly about half of them are a single tab).
I am addicted to tabs and have 260 tabs (yes, I have a problem) open and running smoothly in Firefox. That was not possible in Chromium last time I used it.
Firefox currently uses about 3GB memory, which means I have plenty of memory left on my computer. So I do not think memory usage in the general case is so bad for Firefox and neither is scalability or performance. The problem Firefox historically has had and still to a degree has is lack of responsiveness.
20 represents an extreme slimming down for me. I usually run somewhere between 80 and 200 spread across a couple of windows, but can often go above. I installed a tab counter in FF just for fun. I'm not sure what site you have open in your one FF tab, but it must be a very specific one.