Firefox not wroking well on Ubuntu Linux

Do you see any spikes in CPU or memory usage when you scroll the page?

I'd suspect your video driver. What graphics card do you have in your machine?

Regards, Tobias

These ae the specification for the video card I have:

ATI Technologies               FireMV™ 2200 card

Hardware • Intel® Pentium® 4/Xeon®; AMD Athlon® 64/K7/XP or
compatible.
• 128 MB of system memory; 256 MB or more for best
performance.
• Motherboard with available X16 lane PCI Express™
slot and correct chipset driver or available PCI slot, as
appropriate.
• CD-ROM drive (required for installation software).
Operating
System
• Windows® 2000 with Service Pack 1 (or higher).
• Windows® XP Home or Pro with Service Pack 1 (or
higher).
Monitor • High-resolution MultiSync or multi-frequency monitors
or other VGA monitors.
• Digital flat-panel (DFP) displays or digital CRT
displays.

It works fine in windows XP but I also believe this could be the problem in Linux. I have the CD with drivers for windows but don't have any related to Linux.
My computer is a DELL 520 with 2GB ram.

I don't see any bug spikes or changes under system monitor when I scroll.

Another thing is that the problem happens specially when I have windows maximized

Well, you have a large monitor (25"), hence possibly a pretty large standard screen resolution. Maximize a window under such conditions and then scroll: that is exactly where a gfx driver that is either not the correct one for the hardware its running on, or badly programmed, will start to stutter.

In your last thread, arnold linked to the original ATI drivers that should work for your hardware (https://a248.e.akamai.net/f/674/9206/0/www2.ati.com/drivers/linux/ati-driver-installer-8.25.18-x86.run); in your closing remark you then said that you had found another driver which enhanced performance. Obviously, none of these did the trick in the end, otherwise you would not have started another question on the same topic.

You could certainly try to sort out your drivers following the Ubuntu documentation. Check the two links I provided above:
https://help.ubuntu.com/community/BinaryDriverHowto
https://help.ubuntu.com/community/BinaryDriverHowto/ATI

But most of what is said here applies to clean installations, I'm afraid; there is no guarantee it will work just as well for a dist-upgraded installation with its possible mess of old and new, generic and proprietary drivers plus possible remnants of all of these somewhere in the system.

You did not yet comment on the performance of a live CD.

After booting with the CD the overlapping problem was gone, and so did the scrolling is just slower than when booting from the hard drive; at this point I will install from scratch and load the drivers again. I will keep you posted.