What kc said, Sandra is my constant companion. Lets you stress one item at a time.
I know exactly what you mean on the current problem. The lag out happens to me in just about all applications from word processor to games. It just doesn't happen often enough to really annoy me. The delay is usually accompanied by heavy disk activity.
Make sure your HD is not powering down for battery saving and then having to spool up to cache something from ram. Just to see if it helps - try limiting your background programs to free up ram, turn off disk cacheing of ram, change the time delay on HD power down if you can.
A recent discovery - the windows helpsrv.exe has started spazing out and consuming 100% of CPU capacity on some peoples systems sometime last summer. Windows has a hotfix but it's on their call and pay service. An alternate is to change helpsrv from auto default to manual or kill it for a test(you probably want it on).
The more usual suspects - background app's/malware stealing compute cycles
The newer breed of suspects - you have an X2... The kids could be arguing over who is in charge.
If firefox is the only app with the lag issue - has firefox started incorporating any multithreading? They tend to be first adopters. Try an older version.
The old "it's your drivers story" does piss one off but. It's what ATI told me and after one RMA and 4 months of multiple new sound, video and MB drivers my vid card performance is now tolerable (still wish I bought an Nvidia but, sigh). With something new like X2, people adding multithread support and MS constantly making changes driver stability is an issue.
As for those about to jump in with "I have an X2 and it is completely stable". Good for you, I'm glad. But don't discount other people having issues. Software operation is path dependent. Hardware lot variation, software Load and execution order can cause instability in a varying percentage of products. I spend 50 hours a week finding those kinds of problems and they always do exist. Pin pointing the source is what gives me my rosey disposition.