I'm finding the opposite...
Firebird on a beige G3 233 running 10.2.6 and it's plenty quick for me. I'd say, aside from starting the app, it's as quick as Safari 1.0, but still not as quick as OmniWeb.
the only problem, is that I'm comparing against browsers you can't run =^(
Mozilla 1.3 takes roughly 24 whole seconds to launch on my 500MHz G3 iMac... I'll use it as the benchmark. And it takes nearly a minute to start on the G3 233.
Firebird launches in 15 seconds on the 500MHz G3, while it launches in about 30 seconds on the 233.
just a rough benchmark. your results may vary, especially considering OS reqs and overhead