Everything you are saying points to a problem with the internet
connection rather than the computer itself so have you checked what
download speed you are getting ? www.speedtest.net is a good site to
bookmark for this :-)

I'm assuming that other use of the computer is fast enough, i.e.
opening documents and photos etc ?

A failing hard drive will make a computer slow no matter what operating
system is installed, or how good your internet connection is. I would
run Hirens ultimate boot cd, DOS programs, Hard drive tools, HDAT2 and
check it for bad sectors.

look just run disk utility from software centre that will tell you if you have
any bad sectors

I used to use a Dell Dimension 2400 too, but with only 1 GB RAM. I had a lot of
trouble with the Intel Integrated video when running Ubuntu 8.10, 9.04, 9.10,
and 10.04 LTS. 10.10 ran perfectly; I retired that machine before ever trying
11.04 or 11.10.

My first thought was that you were having trouble with that obsolete video
subsystem. But, upon re-reading your post, I'm more inclined to think that
having that many tabs open in Firefox is crushing your CPU. You might try to
keep a terminal window open, running the "top" command, and watching tasks and
CPU usage as you open up additional tabs. (I've got a still-older and slower
laptop, running Xubuntu 11.10, that will exhibit the behavior you describe with
only three or four tabs open in Firefox.)

This is really interesting. How do I run the "top" command? How can I monitor
CPU usage with Ubuntu? I know how to use the Task Manager in XP but no idea
with Ubuntu? Is there a longer range solution with fixing the Intel video? I
had problems with XP running slow when I had XP.

g> How do I run the "top" command?

From a terminal window, just type the command and press [Enter] to
run.

g> How can I monitor CPU usage with Ubuntu?

The same information that 'top' displays is available by running
System Monitor. You can kill processes with a right click on one just
the same as WinXP task manager - but as with that, be careful what you
stop

My computer went slow for a time tried everything then tried disk utility from
software centre it said the drive had bad sectors .
I replaced the drive and now its back to being fast running same software and it
rockets through them.
the drive was slowing the system down now its a paperweight.

Seen that many times over the years. Lethargic PC that doesn't respond
to any fix - including an OS re-install. Swap the HD for a new one and
it's like letting the brakes off

I will definitely check on getting a new HD for my computer.
Thanks for the information.