Desktop dims when Thunderbird and Firefox are launched

I am running Ubuntu 12.04 (almost default install regularly updated) Unity interface on ASUS X53U (AMD Brazos Dual Core C60 with 2GB RAM).

On launching Thunderbird and Firefox, the application dims and the cursor changes to wait mode. In case of Thunderbird, this is most pronounced with the wait time of up to a minute.

Memory status checked with free indicates around 500MB of free memory on such occasions. The OS is stable and I can switch to a different work-space, etc.

What could this be? Is this something normal?

Why dimming?

The window dims because of a compiz setting , which dims the windows of programs which are not responding.

You can change the setting, if you have compizconfig-settings-manager installed.
Open it and go to Fading Window plugin. There is a settings named Dim unresponsive windows. Uncheck the setting if you don't want the dimming. But I don't recommend it, since it tells you which program is not responding.

Why Not responding?

The application may seem to be not responding while starting up sometimes. Mostly it happens when it does some background activities, such as fetching settings, loading files and trying to connect to the Internet etc.

I think, the most likely cause in your case is, You get large number of new messages in your email account everyday and thunderbird was trying to fetch those emails. But It is not a big problem since it quickly becomes responsive after the operation. It may also related with your Internet connection speed, which may be slowing down fetching and syncing information with your email account.

Applications "dim" when they are busy like loading from disk. This is common on slower computers where applications might be competing for resources, or launching a bunch of applications at once.

This is normal on lower performance Notebooks. You might want to consider using a lighter desktop like Lubuntu or Unity 2d:

• How do I install LXDE / Lubuntu?
• How do I install and switch to the Unity 2D desktop?

I have experienced similar issues, but mostly with Thunderbird.

See:

Why is Thunderbird pegging a core at 100%?

Try disabling Thunderbird add-ons, as you (like me) are probably not using them anyway.