Firefox 4 launches firefox-bin but does not open up GUI nor does it do anything else. Ubuntu 11.04. Also happened on Ubuntu 10.10.

Since 4.0 in 10.10 wasn't installed from official repos it needed to be removed, otherwise links may not be updated properly and conflict with default 4.0 from upgrade to 11.04.
In case remove 4.0 which remains from 10.10, backup profile (move ~/.mozilla), reinstall 4.0 from official repo or maybe only reconfigure helps.
dpkg-reconfigure firefox

https://support.mozilla.com/en-US/kb/Backing%20up%20your%20information
https://support.mozilla.com/en-US/kb/Profiles#w_what-information-is-stored-in-my-profile

## Unity-session
Example of .desktop file.
http://paste.ubuntu.com/614335/

Verify
system-settings -> preferred applications -> web browser = firefox
gconf-editor
/desktop/gnome/url-handlers/http = firefox %u

Well, I can't seem to find that system-setting place you mention. :P

I have completely removed every firefox thing from 11.04 and re-installed it from the official repository. Same thing. Opens the process, firefox-bin, using 22,4MB of RAM and a (I assume glitched) 96% of CPU usage. :P Nothing happens, no error message no nothing. But if I try to start if again, it recognizes that another firefox session is open.

If you think that what you told me to verify would be helpful, please consider me a Linux newbie. :)

Ok, news! I just purged firefox and re-installed (again) through the terminal. It opens, but then freezes, giving this message:

seltor@SeltorLNX:~$ firefox

(firefox-bin:3176): LIBDBUSMENU-GTK-CRITICAL **: dbusmenu_menuitem_property_set_shortcut: assertion `gtk_accelerator_valid(key, modifier)' failed

Don't think the message is related to the freeze, it just tries to set an invalid shortcut somewhere. If you search the error it appears often in users .xsession-errors (hidden file in /home/user).

Options to find system-settings.
- Click on session-menu in top panel, last entry: system-settings
- Open the dash: super+a, start typing sys...

https://help.ubuntu.com/11.04/ubuntu-help/index.html
http://askubuntu.com/questions/28086/keyboard-shortcuts-in-unity

Open a terminal: ctrl+alt+t
Post output of:
gconftool-2 -R /desktop/gnome/url-handlers/http

Anyway, while debugging FF GUI should be closed.
Run FF in safe mode via terminal to see if it freezes without extension+plugin stuff.
firefox -safe-mode

https://support.mozilla.com/en-US/kb/Safe%20Mode

Move (backup) your profile to restart FF with a clean profile. Bookmarks and else can be reimportet later successively to trigger cause of freeze.

Terminal:
mv ~/.mozilla mozilla-backup

## The -backup directory is then (unhidden) in your /home/user

Restart FF to test if it still freezes.

Well Sam, thanks a lot for your help!! Unfortunately, I decided to take the "fresh install" route, since it was giving me other problems other than firefox. Works like a charm now. :P

Again, thank you so much and sorry for wasting your time...