Firefox was just an example, I experience poor responsiveness pretty much everywhere. The bugs that Dylan linked are quite probably related, this comment is quite worrying (the poster did a lot of work on CPU scheduling in the kernel, he knows what he's talking about)
https://bugzilla.kernel.org/show_bug.cgi?id=13347#c59
This thread is basically describing that Vista and Windows 7 feel a lot more responsive than Ubuntu does, that's not right. If I'm running updates in the background I shouldn't have to wait for them to finish before I can watch a YouTube video (for example) and that's the situation I've seen on all my PCs (work and home, desktops and laptops). My text editor shouldn't stutter when installing applications.
I'm trying to describe what is a general issue, but the majority of instances may well stem from that kernel bug.
Luke.