The facts are real simple-
I use Firefox tabbed browsing.
I have a 2.8ghz P4 Prescott with 1GB Ram 2 -10,000 RPM drives -it is a fast computer- and high speed internet connection.
Despite the difference in computers, have seen this occur as well.
I believe it is a result of the tabs loading the multiple pages and mismanaging the process as the load increases.
It could be a bug or it could just be some code that needs to be rewritten to use less resources.(kinda sounds like the same thing)
The problem here is not your computer.
However if you find the computer to be generally slow- i am not surprised. WIndows 2000, ME, and XP generally don't run well with less than 128MB RAM. Infact I suggest 256mb-512mb for best bang for the buck.
FYI
Celeron CPU's are very slow compared to Pentium 3's, Pentium 4's, and Athlon's.
HD performance is determined more by the rotational speed than the size. Faster is better. Laptops have 4200rpm drives and 5400rpm drives. For desktops 5400rpm is slowest, 7200rpm is better. There are faster drives but they are not the norm in machines like yours.
Good question- well written.
Good first steps.
You don't mention Antivirus. I find AVG tends use less overhead than Mcafees and Norton. - and it is free. It works great others swear by it too.
I think you are on the right track but can't fix this. It may be fixed in later versions. Remember this is a beta product still.
I wouldn't put money into a system with Celeron 400- it will only go so fast(slow). Save up for new machine.