Debian – Why does Stackoverflow in Iceweasel cause high CPU usage for Xorg

This is a bug, either in an application that's running on your X server, or in the X server and triggered by an application you're running, or (most likely) in your display driver.

To pinpoint the culprint, start an X server that's not running any risky application. Log out, then switch to a text console by pressing Ctrl+Alt+F1. Log in, then run sudo service gdm stop and startx xterm -e top -- :0. If the CPU load increases, then your driver is the culprit; otherwise it could be either an application making repeated requests to the X server or a bug in the X server. If you can't see any application using up CPU time, the X server is likely to be the sole culprit.

Once you've done a little bit of investigation, report the bug to your distribution. (Search first to see if someone has already reported it.) For Ubuntu, look at How do I report a bug?. Include the line in the output of lspci for your video card.

As others have explained, the desktop meta-packages — such as task-desktop or gnome-core — install a web browser nowadays (well, for quite a long time actually). You might expect gnome-core to install Epiphany, or at least allow it as an alternative to Iceweasel, but it doesn't for security reasons. The gnome-core description mentions the browser dependency:

These are the core components of the GNOME Desktop environment, an intuitive and attractive desktop.

This meta-package depends on a basic set of programs, including a file manager, an image viewer, a web browser, a video player and other tools.

It contains the official “core” modules of the GNOME desktop.

So the reasons it depends on Iceweasel are two-fold:

• it's defined as depending on a web browser;
• the only sensible browser to depend on for the GNOME desktop is Iceweasel, because Epiphany doesn't have enough security support, and Chromium doesn't integrate into the desktop properly.

There used to be an alternative dependency on gnome-www-browser, but that was removed in 2011 (without explanation as far as I can tell). It may be worth asking the maintainers to re-introduce it, but it wouldn't help you install gnome-core without a browser.

The mechanisms which lead to GNOME being removed if you remove Iceweasel are relatively straightfoward. When you ask apt-get to do something, it tries really hard to do it — so removing a package removes anything which depends on it (after asking you). gnome-core depends on iceweasel, and gnome depends on gnome-core, so

apt-get remove iceweasel

also removes gnome-core and gnome.

Removing these meta-packages causes all the packages they depend on to become candidates for removal using autoremove, since the packaging system now considers them to be unnecessary (no package marked as not automatically installed depends on them). The packaging system considers that the user really wants those packages which are marked as explicitly installed, and anything else is only installed to support those packages. So if anything removes gnome or gnome-core, the next time you run apt-get autoremove, it will consider that many of the installed packages are unnecessary...

There are a couple of workarounds:

• if you want to keep gnome-core installed without Iceweasel, use equivs or apt-holepunch (the latter is much easier to use in this case, thanks Joshua!) to build a fake iceweasel package and install that along with gnome-core;

• go through all the packages that gnome and gnome-core depend on, decide which of them you want to use and/or need (e.g. gdm3, gnome-session, nautilus...), and mark them using

    apt-mark manual ...
  

  or using aptitude's GUI (which will be a lot easier).

In any case you can't break your VM by removing packages unless you start removing essential packages (and apt-get will loudly complain before letting you do so), or the kernel. You might end up having to log in to a text console, but you can fix things from there just as well as from an X terminal emulator.