When I'm on Facebook and I'm writing something (like a status or a comment), letters appear very slowly and sometimes the browser freezes while I'm typing.
It seems like the browser is constantly reloading the page, slowing down the whole computer.

This happens only when I'm using Opera (works fine with Iceweasel or Chrome), which is fast and stable doing other stuff (like watching flash videos, surfing the internet..) but nearly unusable on Facebook.

I'm using Debian Testing XFCE, my specs are below.

EDIT: Forgot to say that this does not happens only with this PC, but also on my netbook, which uses the same OS and the same browser

I think this might have something to do with Facebook itself. My dad has this problem all the time, and he still uses Windows Vista and IE. I don't use Facebook myself, but one or two family members have logged into it from my own laptop using Firefox and have not mentioned any problems. Are you running anything else at the same time? How much RAM does the other computer have? All I can really think of is that it's a CPU issue. But then, I am relatively new to Linux too.

Both computers have 1 GB of RAM (not much, but enough for basic tasks and running Debian well) whereas the desktop has a Pentium 4 processor and the netbook an Intel Atom...
Not the most powerful pcs in the world, but I don't experience any problem while surfing heavy site like YouTube, Gizmondo or CNET...

I have used Facebook with Opera on Debian many times (I've severely cut back on Facebook logins in the past year).

I have not encountered anything like this, so I also think it's unlikely to be an Opera issue.

Come to think of it, I was using a Knoppix - which is Debian-based - LiveCD for a couple of months, and used Opera with that too. And again, noone mentioned any problems. Maybe something on Facebook clashes with certain slightly older pieces of hardware. Pentium 4s are a few years old now I think, and my dad's laptop is just marginally newer.

Check if Opera Turbo is "on". If it is or if it is on "auto", turn these "off".

Try updating Java and either re-install Opera or reboot at least after an Java update. Or it could be you need mesa or Compiz plugins or settings changed, maybe. Did you install Opera within the package manager or did you download it from Opera's website?

You should read this page very thoroughly and do as it says actually, if you have not already...

http://wiki.debian.org/Opera

Solved enabling cache (20MB) and unchecking the option for deleting them when I close the browser!

Thanks guys

EDIT: I'm still wondering why in the forum I am recognized as a mac user when I use Midori to surf the internet...

That is dependent on your browser's user agent string.

Glad you fixed it and filled us in on the end of the story.